diff --git a/BUILD.md b/BUILD.md
index 0ff0327..478fe0b 100644
--- a/BUILD.md
+++ b/BUILD.md
@@ -57,6 +57,7 @@ the full ~2.8x speedup; the default uses all cores for no extra gain:
     E           ecoregions colour view (names ecological provinces on first use; Eco tab)
     I           habitability heat map (where civilization can thrive)
     U           settlements: the dawn of civilization on first press, then toggle markers (Civ tab)
+    P           territory / realms view + political borders (nations listed in the Realms tab)
     Y           follow-cam: cycle the 3D camera through active storms (Live World; off after last)
     . / ,       step the live clock forward / back by one rate-unit (auto-pauses; back also
                 rewinds weather + storms via an undo history)
@@ -310,6 +311,18 @@ population grows/declines on the Live World clock toward a food-driven carrying
     civStormDeathRate    0.50  /yr deaths from a full-strength storm over a settlement
     civHurricaneDeathMult 3.0  extra storm-death multiplier for a hurricane/typhoon
 
+Territory & nations (PlanetConfig, key P — derived from settlements, not saved): each settlement
+projects a size-scaled influence range; nearby smaller towns become vassals of a larger capital
+(a kingdom/empire), the rest are city-states; land cells inside a settlement's reach are its
+territory, leaving wilderness frontiers between realms.
+
+    civTerritoryBase     0.035 rad  base influence radius of any settlement
+    civTerritoryScale    0.05  rad   extra reach per log10 of population (big cities reach far)
+    civTerritoryMax      0.35  rad   cap on a single settlement's reach
+    civVassalRange       1.5         x a capital's range = how far it annexes towns into its realm
+    civEmpireMinMembers  5           settlements in a realm to count as an empire
+    civEmpirePop         5e6         total realm population to count as an empire
+
 ## Headless logic test (no display)
 
     g++ -std=c++17 -O2 -Isrc/sim test_logic.cpp src/sim/IcoSphere.cpp \
@@ -320,11 +333,11 @@ population grows/declines on the Live World clock toward a food-driven carrying
         src/sim/PlanetBiota.cpp \
         src/sim/PlanetFloraGen.cpp src/sim/PlanetFaunaGen.cpp src/sim/PlanetFungiGen.cpp \
         src/sim/NameGen.cpp src/sim/PlanetGeography.cpp src/sim/PlanetEcoregions.cpp \
-        src/sim/PlanetCiv.cpp \
+        src/sim/PlanetCiv.cpp src/sim/PlanetNation.cpp \
         src/sim/PlanetIO.cpp -o /tmp/t && /tmp/t
 
     # Same source list for every suite: swap test_logic.cpp -> test_biota / test_live / test_ocean /
-    # test_weather / test_volcano / test_geography / test_ecoregions / test_civ.
+    # test_weather / test_volcano / test_geography / test_ecoregions / test_civ / test_nation.
     # The CMake build also includes test_events for the viewer event journal.
 
 Verifies geometry, plate assignment, gradual non-saturating relief and
diff --git a/CLAUDE.md b/CLAUDE.md
index 176ba92..063a93c 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -108,7 +108,7 @@ the fixed-grid Eulerian model + the climate fields are the groundwork for it.
 **Civilizations (in progress — the long arc after the world is finished):** the eventual goal is
 people who eat, name their world, found villages→cities, build kingdoms/empires, draw cultural +
 geographic borders, and go to war. Built in phases (cell = territory, settlements = point agents, all
-on the Live World clock). **Steps 1–2 of the roadmap are done (plus a derived ecoregions atlas):**
+on the Live World clock). **Steps 1–3 of the roadmap are done (plus a derived ecoregions atlas):**
 - **Geography & place-names (the atlas)** *(done — see `PlanetGeography.cpp` + `NameGen.cpp`)* — the
   foundation everything civic references. `Planet::generateGeography()` extracts named features from
   the (frozen) terrain by connectivity over the fixed grid — **continents/islands** (connected land),
@@ -151,8 +151,23 @@ on the Live World clock). **Steps 1–2 of the roadmap are done (plus a derived
   `WorldEvent`s ("X grew into a city", "Hurricane <name> devastates X", "Famine shrinks X to a Town", "X
   was abandoned"). The set is fixed, so the step-back snapshot only restores the per-settlement
   **population** vector; conditions recompute. Saved (**v20**). Knobs `civ*`.
-  *Next steps (not yet built): territory + borders, kingdoms/empires, culture + beliefs, conflict +
-  diplomacy.*
+- **Territory, nations & political borders (Step 3)** *(done — see `PlanetNation.cpp`)* — settlements
+  are grouped into **realms** and claim land. `computeTerritory()` (deterministic, no RNG): each living
+  settlement projects an **influence range** that scales with population (`civTerritory*`); **realm
+  grouping** processes settlements largest→smallest — a settlement joins the nearest larger **capital**
+  whose annexation reach (`civVassalRange`) covers it (a vassal town → kingdom) else founds its own
+  nation; tier = **City-state / Kingdom / Empire** by member count / total pop (`civEmpireMinMembers`/
+  `civEmpirePop`); each land cell goes to the settlement maximising `range − distance` (else
+  **wilderness** −1), so its nation is that settlement's → **influence-limited territory with wilderness
+  frontiers**. Borders trace the per-cell `cellNation()` edges (the plate dual-contour reused as
+  `buildNationBorders`). Render: a **Territory** colour mode (`nationColor`) + dark border lines + realm
+  labels at capitals (key **`P`**), a **Realms** tab, a cell-info realm line, and kind=4 `WorldEvent`s
+  ("The Kingdom of X is founded", "X rises to an Empire", "the X collapsed"). Territory + nations are a
+  **pure function of the (saved) settlements**, so they're **recomputed** (each sim year / on placement /
+  load / step-back) — **no save state, no version bump**, peaceful & population-driven. Knobs `civTerritory*`/
+  `civVassalRange`/`civEmpire*`.
+  *Next steps (not yet built): culture + beliefs + governments (and culture-driven borders/renaming),
+  conflict + diplomacy (war moving borders by force), trade.*
 
 ## Current state
 
@@ -562,6 +577,7 @@ src/
     PlanetGeography.*    generateGeography() (named features: continents/oceans/ranges/rivers/lakes)
     PlanetEcoregions.*   generateEcoregions() (named ecological provinces + dominant biota/productivity)
     PlanetCiv.*          computeHabitability/placeSettlements/stepCivilization (settlements; civ Step 2)
+    PlanetNation.*       computeTerritory (realms + per-cell ownership + borders; civ Step 3)
     PlanetIO.cpp         config file (text) + binary save/load
   render/  (raylib viewer)
     Colors.*          cell color modes (elevation/plate/age/crust/biome/climate/biota)
@@ -625,12 +641,13 @@ g++ -std=c++17 -O2 -Isrc/sim test_logic.cpp src/sim/IcoSphere.cpp \
     src/sim/PlanetBiota.cpp \
     src/sim/PlanetFloraGen.cpp src/sim/PlanetFaunaGen.cpp src/sim/PlanetFungiGen.cpp \
     src/sim/NameGen.cpp src/sim/PlanetGeography.cpp src/sim/PlanetEcoregions.cpp src/sim/PlanetCiv.cpp \
+    src/sim/PlanetNation.cpp \
     src/sim/PlanetIO.cpp -o /tmp/t && /tmp/t
 ```
 (Swap `test_logic.cpp` for `test_biota.cpp`, `test_live.cpp`, `test_ocean.cpp`,
-`test_weather.cpp`, `test_volcano.cpp`, `test_geography.cpp`, `test_ecoregions.cpp` or `test_civ.cpp`
-to run the Biota / Live World / Ocean / Weather / Volcano / Geography / Ecoregions / Civilization
-suites — same source list. CMake also builds `test_events` for the
+`test_weather.cpp`, `test_volcano.cpp`, `test_geography.cpp`, `test_ecoregions.cpp`, `test_civ.cpp` or
+`test_nation.cpp` to run the Biota / Live World / Ocean / Weather / Volcano / Geography / Ecoregions /
+Civilization / Nation suites — same source list. CMake also builds `test_events` for the
 viewer event journal.)
 
 Use this to verify tectonics after changing `Planet::step()` without launching
@@ -666,6 +683,7 @@ all in 3D + 2D) · `N` day/night terminator (Live World) · `T` tide-coloured co
 `V` volcano markers (Live World) · `M` place-name labels (the atlas; names the world on first use) ·
 `E` ecoregions colour view (names ecology on first use) · `I` habitability heat map ·
 `U` settlements (the dawn of civilization on first press; toggles markers after) ·
+`P` territory / realms view + political borders (Realms tab lists nations) ·
 `SPACE` or on-screen button pause ·
 `[`/`]` drift speed (My/sec) — in **Live World** the live-clock rate (hours/sec, hour→month) ·
 `S` single tick (in **Live World** steps the clock forward) · `.`/`,` step the live clock
@@ -879,6 +897,13 @@ triangles (plates are fixed in phase 1).
   `civHurricaneDeathMult` (3.0, deaths from a storm/hurricane over a town). Droughts/harvests are
   deterministic per (~20° region, year, seed); an active volcano's ash within ~1.5× its blast radius also
   cuts capacity. Marker sizes/colours + hardship tint are render constants (ViewerRender.cpp).
+- **Territory & nations (`civTerritory*`/`civVassal*`/`civEmpire*`, `planet.cfg`; key `P`):**
+  `civTerritoryBase` (0.035 rad, a village's reach), `civTerritoryScale` (0.05 rad per log10 of
+  population/seed — big cities reach far), `civTerritoryMax` (0.35 rad cap); `civVassalRange` (1.5 ×
+  a capital's range = its annexation reach for vassal towns → bigger = larger kingdoms); empire
+  threshold `civEmpireMinMembers` (5 settlements) / `civEmpirePop` (5e6 total). Territory + realms are
+  **derived** (recomputed each sim year, not saved). Realm colours/border colour/labels are render
+  constants (Colors.cpp / ViewerRender.cpp).
 - `upliftGain` (PlanetConfig) — m/tick per unit convergence stress; main
   knob for how fast/high relief builds.
 - `relax` (PlanetConfig) — isostatic relaxation toward base elevation. Peaks
diff --git a/CMakeLists.txt b/CMakeLists.txt
index cfbf3be..e6b2caa 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -36,6 +36,7 @@ set(SIM_SOURCES
     src/sim/PlanetGeography.cpp
     src/sim/PlanetEcoregions.cpp
     src/sim/PlanetCiv.cpp
+    src/sim/PlanetNation.cpp
     src/sim/PlanetIO.cpp
 )
 
@@ -74,7 +75,7 @@ if(UNIX AND NOT APPLE)
 endif()
 
 enable_testing()
-foreach(test_name logic biota ocean live weather volcano geography ecoregions civ)
+foreach(test_name logic biota ocean live weather volcano geography ecoregions civ nation)
     add_executable(test_${test_name} test_${test_name}.cpp)
     target_link_libraries(test_${test_name} PRIVATE planetsim_sim)
     add_test(NAME ${test_name} COMMAND test_${test_name})
diff --git a/docs/design-notes.md b/docs/design-notes.md
index cbaf14d..7c7418f 100644
--- a/docs/design-notes.md
+++ b/docs/design-notes.md
@@ -52,6 +52,7 @@ include path, so includes stay flat (`#include "Planet.hpp"`, `"Viewer.hpp"`).
   land/water context, productivity and broad biota; saved v19).
 - `PlanetCiv.{hpp,cpp}` — `computeHabitability`/`placeSettlements`/`stepCivilization` (civ Step 2:
   habitability + settlements that grow/decline on the live clock; saved v20).
+- `PlanetNation.{hpp,cpp}` — `computeTerritory` (civ Step 3: realms + per-cell ownership; derived, not saved).
 - `PlanetIO.cpp` — text config + binary save/load.
 
 The viewer is one `Viewer` struct: `Viewer.{hpp,cpp}` (state + setup + sim orchestration),
@@ -398,6 +399,27 @@ a cell-info line, and a `Habitability` colour mode (key `I`). `buildGeometry()`
 Save **v20** stores the settlement records (population included); `sCellSettlement` is rebuilt on load.
 Knobs `civ*`.
 
+## Civilization Step 3 — territory, nations & political borders
+
+`PlanetNation.cpp` (engine, raylib-free, **no RNG** → tectonic stream untouched). `computeTerritory()`
+turns the settlement set into **realms** + per-cell ownership, all a **pure deterministic function of
+the (saved) settlements** — so it is *recomputed*, never saved (no `SAVE_VERSION` bump), and step-back
+replays it as populations restore. (1) **Influence range** per living settlement scales with population
+(`civTerritory*`). (2) **Realm grouping**: process settlements largest→smallest; a settlement joins the
+nearest larger **capital** whose annexation reach (`civVassalRange × its range`) covers it (→ a vassal
+town of that kingdom) else founds its own nation; tier City-state / Kingdom / Empire by member count /
+total pop. (3) **Per-cell ownership**: each land cell goes to the settlement maximising `range −
+angular-distance` if > 0, else **wilderness** (−1) — influence-limited, with frontiers; the cell's
+nation is that settlement's. Peaceful + population-driven (no conquest yet).
+
+Render: a `Territory` colour mode (`nationColor`, golden-ratio HSV) + dark **border lines**
+(`buildNationBorders` — the plate dual-contour reused, keyed on `cellNation()`) + realm labels at
+capitals (key `P`), a **Realms** tab (8th), a cell-info realm line, and **kind=4** `WorldEvent`s
+(realm founded / rises to an empire / collapsed — detected in `detectNationEvents` by matching nations
+across a recompute by capital id). The viewer recomputes territory + rebuilds borders **once per sim
+year** (`liveAdvance` year-tick), and on placement / load / step-back; `buildGeometry()` clears
+`nations`/`sCellNation`/`sSettleNation` on reseed. Knobs `civTerritory*`/`civVassal*`/`civEmpire*`.
+
 ## Headless testing
 
 Engine is raylib-free, so logic is tested without a display. Build/run:
diff --git a/src/render/Colors.cpp b/src/render/Colors.cpp
index fc71f94..b0e9451 100644
--- a/src/render/Colors.cpp
+++ b/src/render/Colors.cpp
@@ -93,6 +93,7 @@ const char* colorModeName(ColorMode m) {
         case ColorMode::FungaDensity: return "Funga density";
         case ColorMode::Ecoregion:    return "Ecoregions";
         case ColorMode::Habitability: return "Habitability";
+        case ColorMode::Territory:    return "Territory / realms";
         case ColorMode::TempSummer:   return "Temperature (summer)";
         case ColorMode::TempWinter:   return "Temperature (winter)";
         case ColorMode::Seasonality:  return "Seasonality (summer-winter)";
@@ -142,6 +143,12 @@ Color marineFaunaColor(double d01) {   // deep blue -> cyan -> warm (rich shelve
     return Color{ L(0), L(1), L(2), 255 };
 }
 
+Color nationColor(int id) {             // distinct per-realm tint (offset hue/sat vs plateColor)
+    if (id < 0) return Color{ 40, 44, 50, 255 };   // wilderness: dim grey
+    float h = std::fmod((id + 4) * 0.61803398875f + 0.13f, 1.0f) * 360.0f;
+    return ColorFromHSV(h, 0.58f, 0.92f);
+}
+
 Color habitabilityColor(double h01) {   // barren grey -> green -> fertile gold
     double t = std::clamp(h01, 0.0, 1.0);
     static const unsigned char key[3][3] = {
diff --git a/src/render/Colors.hpp b/src/render/Colors.hpp
index bc6f446..085b421 100644
--- a/src/render/Colors.hpp
+++ b/src/render/Colors.hpp
@@ -6,7 +6,7 @@
 
 enum class ColorMode { Elevation, Plate, Age, Crust, Biome, Temperature, Precip,
                        FloraDensity, FaunaDensity, FungaDensity,
-                       Ecoregion, Habitability,
+                       Ecoregion, Habitability, Territory,
                        TempSummer, TempWinter, Seasonality };  // 6 cycles these temp sub-views
 
 Color elevationColor(double e, double seaLevel);
@@ -42,3 +42,5 @@ Color marineFaunaColor(double d01);
 Color ecoregionColor(int id, Biome b, double productivity);
 // Habitability heat map (0..1): barren grey -> fertile green/gold (where civilization can thrive).
 Color habitabilityColor(double h01);
+// Per-nation territory tint (golden-ratio HSV, offset from plateColor so realms read distinctly).
+Color nationColor(int id);
diff --git a/src/render/Overlays.cpp b/src/render/Overlays.cpp
index 4f832ca..cc37857 100644
--- a/src/render/Overlays.cpp
+++ b/src/render/Overlays.cpp
@@ -41,6 +41,34 @@ void buildBorders(const Planet& p, float radius,
     }
 }
 
+void buildNationBorders(const Planet& p, float radius, std::vector<Vector3>& segs) {
+    segs.clear();
+    const std::vector<int>& cn = p.cellNation();
+    if ((int)cn.size() != (int)p.cells.size()) return;
+    auto midV = [&](int i, int j) -> Vector3 {
+        Vec3 m = ((p.cells[i].unit + p.cells[j].unit) * 0.5).normalized() * radius;
+        return Vector3{ (float)m.x, (float)m.y, (float)m.z };
+    };
+    auto emit = [&](const Vector3& a, const Vector3& b) { segs.push_back(a); segs.push_back(b); };
+    const std::vector<int>& tri = p.triIndices();
+    for (size_t k = 0; k + 2 < tri.size(); k += 3) {
+        int ia = tri[k], ib = tri[k + 1], ic = tri[k + 2];
+        int na = cn[ia], nb = cn[ib], nc = cn[ic];
+        if (na == nb && nb == nc) continue;
+        if (na != nb && nb != nc && na != nc) {
+            Vec3 c = ((p.cells[ia].unit + p.cells[ib].unit + p.cells[ic].unit) * (1.0 / 3.0)).normalized() * radius;
+            Vector3 C{ (float)c.x, (float)c.y, (float)c.z };
+            emit(C, midV(ia, ib)); emit(C, midV(ib, ic)); emit(C, midV(ic, ia));
+        } else {
+            int lone, o1, o2;
+            if      (na == nb) { lone = ic; o1 = ia; o2 = ib; }
+            else if (nb == nc) { lone = ia; o1 = ib; o2 = ic; }
+            else               { lone = ib; o1 = ia; o2 = ic; }
+            emit(midV(lone, o1), midV(lone, o2));
+        }
+    }
+}
+
 void buildDriftArrows(const Planet& p, float radius,
                       std::vector<Vector3>& out, std::vector<PlateLabel>& labels) {
     out.clear(); labels.clear();
diff --git a/src/render/Overlays.hpp b/src/render/Overlays.hpp
index 887e7ab..f002504 100644
--- a/src/render/Overlays.hpp
+++ b/src/render/Overlays.hpp
@@ -14,6 +14,11 @@
 void buildBorders(const Planet& p, float radius,
                   std::vector<Vector3>& real, std::vector<Vector3>& ridge);
 
+// ---- Nation borders (civ Step 3) -------------------------------------------
+// Same dual-contour, but separating cells of different `cellNation()` (a realm's outline:
+// inter-realm borders + its coast + wilderness frontier). One segment list.
+void buildNationBorders(const Planet& p, float radius, std::vector<Vector3>& segs);
+
 // ---- Per-plate drift arrows -------------------------------------------------
 struct PlateLabel { int id; Vector3 pos; };
 void buildDriftArrows(const Planet& p, float radius,
diff --git a/src/render/Panels.cpp b/src/render/Panels.cpp
index 3af78a5..d1141e8 100644
--- a/src/render/Panels.cpp
+++ b/src/render/Panels.cpp
@@ -105,6 +105,17 @@ static std::vector<std::string> cellInfo(const Planet& p, int i, double elev, do
             }
         }
     }
+    // Territory: which realm controls this cell (civ Step 3).
+    if (p.nationsBuilt()) {
+        const auto& cn = p.cellNation();
+        int ni = (i < (int)cn.size()) ? cn[i] : -1;
+        if (ni >= 0 && ni < (int)p.nationList().size()) {
+            const Nation& nat = p.nationList()[ni];
+            L.push_back(std::string("realm: ") + nat.name + " (" + nationTierName(nat.tier) + ")");
+        } else if (p.cells[i].elevation > p.cfg.seaLevel) {
+            L.push_back(std::string("realm: wilderness"));
+        }
+    }
     // Climate (derived; present once computeClimate() has run).
     if (sized(p.temperature()) && sized(p.moisture()))
         L.push_back(std::string(TextFormat("temp %.1f C   precip %.0f%%",
diff --git a/src/render/Viewer.cpp b/src/render/Viewer.cpp
index 1df1753..ecbf13f 100644
--- a/src/render/Viewer.cpp
+++ b/src/render/Viewer.cpp
@@ -122,6 +122,7 @@ void Viewer::recolor() {
     const std::vector<int>& ecoCell = planet.cellEcoregion();
     const auto& eco = planet.ecoregions();
     const std::vector<double>& hab = planet.habitability();
+    const std::vector<int>& cnat = planet.cellNation();
     vcolors.resize(planet.cells.size());
     for (size_t i = 0; i < planet.cells.size(); ++i) {
         switch (mode) {
@@ -160,6 +161,13 @@ void Viewer::recolor() {
                               && planet.cells[i].biome != Biome::Ice)
                            ? habitabilityColor(hab[i]) : Color{30, 42, 64, 255};   // ocean/ice: dim blue
                 break;
+            case ColorMode::Territory: {
+                int ni = (i < (int)cnat.size()) ? cnat[i] : -1;
+                if (ni >= 0) vcolors[i] = nationColor(ni);                         // owned: realm tint
+                else vcolors[i] = (planet.cells[i].elevation > planet.cfg.seaLevel) // wilderness land vs sea
+                                ? Color{60, 64, 58, 255} : Color{26, 34, 52, 255};
+                break;
+            }
             default:               vcolors[i] = elevationColor(planet.cells[i].elevation, planet.cfg.seaLevel);
         }
     }
@@ -389,6 +397,35 @@ void Viewer::detectLiveEvents(const std::vector<WeatherSystem>& beforeStorms,
     }
 }
 
+// Nation/realm events (kind=4): compare the new realms to the pre-recompute set by capital settlement id
+// -> realm foundings, tier rises (to a kingdom/empire), and collapses (capital lost/absorbed).
+void Viewer::detectNationEvents(const std::vector<Nation>& before) {
+    auto byCapital = [](const std::vector<Nation>& v, int cap) -> const Nation* {
+        for (const Nation& nn : v) if (nn.capital == cap) return &nn; return nullptr;
+    };
+    for (const Nation& nat : planet.nationList()) {
+        if (nat.capital < 0 || nat.capital >= (int)planet.settlements.size()) continue;
+        int cell = planet.settlements[nat.capital].cell;
+        const Nation* o = byCapital(before, nat.capital);
+        if (!o) {
+            if (nat.tier != NationTier::CityState)            // skip lone city-state spam
+                appendEvent(4, 1, liveTime, cell, nat.id, std::string("The ") + nat.name + " is founded",
+                            std::string(TextFormat("%d settlements, pop %.0fk", nat.members, nat.totalPop / 1.0e3)));
+        } else if ((int)nat.tier > (int)o->tier) {
+            appendEvent(4, 1, liveTime, cell, nat.id, nat.name + " rises to " +
+                        (nat.tier == NationTier::Empire ? "an Empire" : "a Kingdom"),
+                        std::string(TextFormat("%d settlements", nat.members)));
+        }
+    }
+    for (const Nation& o : before) {
+        if (o.tier == NationTier::CityState) continue;
+        if (!byCapital(planet.nationList(), o.capital) &&
+            o.capital >= 0 && o.capital < (int)planet.settlements.size())
+            appendEvent(4, 2, liveTime, planet.settlements[o.capital].cell, o.id,
+                        std::string("The ") + o.name + " collapsed", "");
+    }
+}
+
 void Viewer::focusCell(int idx, const std::string& status) {
     if (idx < 0 || idx >= (int)planet.cells.size()) return;
     selectedCell = idx;
@@ -595,6 +632,7 @@ void Viewer::loadGame(const char* path) {
     buildBorders(planet, borderR, borders, ridgeBorders);
     buildDriftArrows(planet, driftR, driftArrows, plateLabels);
     buildMap2D(planet, mapRect, map2D);
+    if (planet.settlementsPlaced()) rebuildTerritory();   // territory/nations are derived -> recompute
     refreshView();
     setStatus(skippedHistory ? std::string("Loaded ") + path + " (history skipped)"
                              : std::string("Loaded ") + path);
@@ -677,11 +715,31 @@ void Viewer::liveAdvance(double dtClock, double dtWeather) {
     VolcanoUpdate vu = planet.stepVolcanoes(dtWeather);
     CivUpdate cu = planet.stepCivilization(dtWeather, liveTime);   // env-driven growth/decline on the clock
     if (dtWeather > 0.0) detectLiveEvents(beforeStorms, beforeVolcanoes, beforeSettlements);
+    // Territory & nations shift slowly -> recompute once per sim year (and rebuild the border lines).
+    bool territoryChanged = false;
+    if (planet.settlementsPlaced()) {
+        double yearHours = std::max(1.0, planet.cfg.dayLengthHours * planet.cfg.yearLengthDays);
+        long year = (long)std::floor(liveTime / yearHours);
+        if (year != lastTerritoryYear) {
+            std::vector<Nation> beforeNations = (dtWeather > 0.0) ? planet.nationList() : std::vector<Nation>{};
+            rebuildTerritory();
+            if (dtWeather > 0.0) detectNationEvents(beforeNations);
+            territoryChanged = true;
+        }
+    }
     if (vu.breach)      refreshView();
-    else if (vu.recolor || cu.recolor) recolor();
+    else if (vu.recolor || cu.recolor || (territoryChanged && mode == ColorMode::Territory)) recolor();
     rebuildLiveOverlay();
 }
 
+// Recompute realms/territory from the (derived) settlement set + rebuild the nation-border segments.
+void Viewer::rebuildTerritory() {
+    planet.computeTerritory();
+    buildNationBorders(planet, borderR, nationBorders);
+    double yearHours = std::max(1.0, planet.cfg.dayLengthHours * planet.cfg.yearLengthDays);
+    lastTerritoryYear = (long)std::floor(liveTime / yearHours);
+}
+
 // Push the current (pre-advance) weather state onto the bounded step-back ring.
 void Viewer::wxPushSnapshot() {
     if ((int)wxUndo.size() >= wxUndoMax) wxUndo.erase(wxUndo.begin());
diff --git a/src/render/Viewer.hpp b/src/render/Viewer.hpp
index 193d684..7997fb9 100644
--- a/src/render/Viewer.hpp
+++ b/src/render/Viewer.hpp
@@ -101,7 +101,10 @@ struct Viewer {
     bool showVolcanoes = true;      // Live World volcano markers (cones + eruption glow, key V)
     bool showNames = false;         // geographic place-name labels (the atlas, key M)
     bool showSettlements = true;    // civilization settlement markers (key U seeds + toggles)
-    std::vector<int> atlasRowCells; // cell to focus per visible Atlas/Eco/Civ-tab row (parallel to the list)
+    std::vector<int> atlasRowCells; // cell to focus per visible Atlas/Eco/Civ/Realms-tab row (parallel to the list)
+    std::vector<Vector3> nationBorders;  // political border segments (rebuilt on year tick / placement / load)
+    bool showNationBorders = false; // draw nation/realm borders (on with the Territory view)
+    long lastTerritoryYear = -1;    // sim year territory was last recomputed (recompute when it ticks)
 
     // World event journal: currently Live World events, shaped to be reused by later phases.
     struct WorldEvent {
@@ -153,6 +156,7 @@ struct Viewer {
     void selectCell(int idx);
     void recolor();
     void refreshView();
+    void rebuildTerritory();             // recompute nations/territory + nation-border segments
     void rebuildLiveOverlay();           // Live World: fill illum + shadedColors from sim fields
     // Colors the 3D globe + 2D map actually draw: the live overlay when in Live World, else the
     // plain per-cell colours.
@@ -175,6 +179,7 @@ struct Viewer {
     void detectLiveEvents(const std::vector<WeatherSystem>& beforeStorms,
                           const std::vector<Volcano>& beforeVolcanoes,
                           const std::vector<Settlement>& beforeSettlements);
+    void detectNationEvents(const std::vector<Nation>& beforeNations);
     void focusCell(int idx, const std::string& status = "");
 
     // ---- Input (ViewerInput.cpp) --------------------------------------------
diff --git a/src/render/ViewerInput.cpp b/src/render/ViewerInput.cpp
index 8ca63c2..cb365f7 100644
--- a/src/render/ViewerInput.cpp
+++ b/src/render/ViewerInput.cpp
@@ -205,6 +205,7 @@ void Viewer::handleInput() {
     if (IsKeyPressed(KEY_U) && settled) {           // civilization: seed on first press ("the dawn"), then toggle markers
         if (!planet.settlementsPlaced()) {
             planet.placeSettlements();
+            rebuildTerritory();                         // initial realms + borders
             showSettlements = true;
             appendEvent(3, 1, liveTime, planet.settlements.empty() ? 0 : planet.settlements[0].cell, 0,
                         "Civilization begins",
@@ -215,6 +216,16 @@ void Viewer::handleInput() {
             setStatus(showSettlements ? "Settlements on" : "Settlements off");
         }
     }
+    if (IsKeyPressed(KEY_P) && settled) {           // toggle the territory / realms colour view + borders
+        if (!planet.settlementsPlaced()) setStatus("Press U for the dawn of civilization first");
+        else {
+            if (!planet.nationsBuilt() || (int)planet.cellNation().size() != (int)planet.cells.size()) rebuildTerritory();
+            mode = (mode == ColorMode::Territory) ? ColorMode::Biome : ColorMode::Territory;
+            showNationBorders = (mode == ColorMode::Territory);
+            recolor();
+            setStatus(mode == ColorMode::Territory ? "Territory / realms on" : "Territory off");
+        }
+    }
     if (IsKeyPressed(KEY_W) && settled) {           // enter / leave Live World (slow real-time clock)
         liveWorld = !liveWorld;
         if (liveWorld) {
diff --git a/src/render/ViewerRender.cpp b/src/render/ViewerRender.cpp
index b40ecd5..784645c 100644
--- a/src/render/ViewerRender.cpp
+++ b/src/render/ViewerRender.cpp
@@ -82,6 +82,14 @@ void Viewer::renderGlobe3D() {
         }
         rlEnd(); rlSetLineWidth(1.0f);
     }
+    if (showNationBorders && !nationBorders.empty()) {     // political / realm borders (dark, over the tint)
+        rlSetLineWidth(2.5f); rlBegin(RL_LINES); rlColor4ub(18, 18, 26, 235);
+        for (size_t i = 0; i + 1 < nationBorders.size(); i += 2) {
+            rlVertex3f(nationBorders[i].x, nationBorders[i].y, nationBorders[i].z);
+            rlVertex3f(nationBorders[i + 1].x, nationBorders[i + 1].y, nationBorders[i + 1].z);
+        }
+        rlEnd(); rlSetLineWidth(1.0f);
+    }
     if (showDrift && !driftArrows.empty()) {
         rlSetLineWidth(2.5f); rlBegin(RL_LINES); rlColor4ub(90, 230, 255, 255);
         for (size_t i = 0; i + 1 < driftArrows.size(); i += 2) {
@@ -335,6 +343,7 @@ void Viewer::renderMap2D() {
     if (showGrat) { drawGraticule2D(graticule, vr, mapLon); drawGraticuleLabels2D(vr, mapLon); }
     if (showBorders && !borders.empty())      drawSegments2D(borders, Color{255, 235, 90, 255}, 2.0f, vr, mapLon);
     if (showBorders && !ridgeBorders.empty()) drawSegments2D(ridgeBorders, Color{220, 70, 60, 255}, 2.0f, vr, mapLon);
+    if (showNationBorders && !nationBorders.empty()) drawSegments2D(nationBorders, Color{18, 18, 26, 235}, 2.0f, vr, mapLon);
     if (showDrift   && !driftArrows.empty()) drawSegments2D(driftArrows, Color{90, 230, 255, 255}, 2.0f, vr, mapLon);
     if (liveWorld && showTides && !coastCols.empty()) drawColoredSegments2D(coast, coastCols, 2.0f, vr, mapLon);
     if (showCurrents && !currentCols.empty()) drawColoredSegments2D(currentSegs, currentCols, 1.6f, vr, mapLon);
@@ -434,10 +443,10 @@ void Viewer::renderLiveInfo() {
     DrawRectangleLinesEx(r, 1, Color{90, 90, 110, 255});
     int x = (int)r.x + 14, y = (int)r.y + 10;
     DrawText("Live info", x, y, 20, RAYWHITE);
-    const char* tabs[7] = { "Sky", "Tides", "Weather", "Events", "Atlas", "Eco", "Civ" };
+    const char* tabs[8] = { "Sky", "Tides", "Weather", "Events", "Atlas", "Eco", "Civ", "Realms" };
     float tx = r.x + 10.0f, ty = r.y + 38.0f;
-    for (int i = 0; i < 7; ++i) {
-        float tw = (r.width - 20.0f) / 7.0f;
+    for (int i = 0; i < 8; ++i) {
+        float tw = (r.width - 20.0f) / 8.0f;
         Rectangle tr{ tx + i * tw, ty, tw - 4.0f, 24.0f };
         liveInfoTabRects.push_back(tr);
         bool on = liveInfoTab == i;
@@ -557,7 +566,7 @@ void Viewer::renderLiveInfo() {
                                            : Color{175, 205, 235, 255};
                 DrawRectangleRec(row, bg);
                 DrawRectangleLinesEx(row, 1, Color{70, 75, 92, 255});
-                const char* icon = e.kind == 2 ? "^" : "~";
+                const char* icon = e.kind == 2 ? "^" : e.kind == 3 ? "*" : e.kind == 4 ? "#" : "~";
                 DrawText(icon, (int)row.x + 7, (int)row.y + 6, 18, fg);
                 double d = e.timeHours / std::max(0.1, planet.cfg.dayLengthHours);
                 DrawText(TextFormat("D%.1f", d), (int)row.x + 24, (int)row.y + 5, 12, Color{145, 155, 175, 255});
@@ -633,7 +642,7 @@ void Viewer::renderLiveInfo() {
                 y += 35;
             }
         }
-    } else {   // Civ: settlements by population (largest first); click a row to fly there
+    } else if (liveInfoTab == 6) {   // Civ: settlements by population (largest first); click a row to fly there
         const auto& S = planet.settlements;
         const double townP = planet.cfg.civTownPop, cityP = planet.cfg.civCityPop, abP = planet.cfg.civAbandonPop;
         DrawText("Settlements", x, y, 18, Color{200, 205, 220, 255});
@@ -665,6 +674,34 @@ void Viewer::renderLiveInfo() {
                 y += 20;
             }
         }
+    } else {   // Realms: nations by population (largest first); click a row to fly to the capital
+        const auto& N = planet.nationList();
+        DrawText("Realms", x, y, 18, Color{200, 205, 220, 255});
+        DrawText(TextFormat("%d", (int)N.size()), (int)(r.x + r.width) - 40, y + 2, 13, Color{145, 155, 175, 255});
+        y += 26;
+        if (N.empty()) {
+            DrawText(planet.settlementsPlaced() ? "press P for the territory view" : "press U then P", x, y, 13, Color{150, 155, 170, 255});
+        } else {
+            std::vector<int> idx(N.size());
+            for (size_t i = 0; i < N.size(); ++i) idx[i] = (int)i;
+            std::sort(idx.begin(), idx.end(), [&](int a, int b) { return N[a].totalPop > N[b].totalPop; });
+            for (int ni : idx) {
+                if (y > (int)(r.y + r.height) - 22) break;
+                const Nation& nat = N[ni];
+                int cap = (nat.capital >= 0 && nat.capital < (int)planet.settlements.size()) ? planet.settlements[nat.capital].cell : -1;
+                Rectangle row{ r.x + 10.0f, (float)y - 2.0f, r.width - 20.0f, 19.0f };
+                eventRowRects.push_back(row); atlasRowCells.push_back(cap);
+                Color fg = nat.tier == NationTier::Empire ? Color{250, 215, 130, 255}
+                         : nat.tier == NationTier::Kingdom ? Color{215, 210, 175, 255}
+                                                           : Color{180, 190, 175, 255};
+                const char* tp = nat.totalPop >= 1.0e6 ? TextFormat("%.1fM", nat.totalPop / 1.0e6)
+                               : nat.totalPop >= 1.0e3 ? TextFormat("%.0fk", nat.totalPop / 1.0e3)
+                                                       : TextFormat("%.0f", nat.totalPop);
+                DrawText(nat.name.c_str(), (int)row.x + 6, (int)row.y + 2, 14, fg);
+                DrawText(TextFormat("%dx  %s", nat.members, tp), (int)(r.x + r.width) - 92, (int)row.y + 3, 11, Color{150, 158, 178, 255});
+                y += 20;
+            }
+        }
     }
 }
 
@@ -743,9 +780,10 @@ void Viewer::renderHUD() {
     line("1 elev  2 plates  3 age  4 crust  5 biome  6 temp* 7 precip  8 flora  9 fauna  0 funga  E eco  (*6 cycles mean/summer/winter/season)");
     line(TextFormat("B borders [%s] | D vectors [%s] | G grid [%s] | J rivers [%s] | N day/night [%s] | T tides [%s] | O currents [%s]",
                     showBorders ? "on" : "off", showDrift ? "on" : "off", showGrat ? "on" : "off", showRivers ? "on" : "off", dayNightOn ? "on" : "off", showTides ? "on" : "off", showCurrents ? "on" : "off"));
-    line(TextFormat("K clouds [%s] | V volcanoes [%s] | M names [%s] | E eco | I habitability | U settlements [%s]",
+    line(TextFormat("K clouds [%s] | V volcanoes [%s] | M names [%s] | E eco | I habitability | U settlements [%s] | P territory [%s]",
                     showClouds ? "on" : "off", showVolcanoes ? "on" : "off", showNames ? "on" : "off",
-                    !planet.settlementsPlaced() ? "seed" : showSettlements ? "on" : "off"));
+                    !planet.settlementsPlaced() ? "seed" : showSettlements ? "on" : "off",
+                    showNationBorders ? "on" : "off"));
     line(TextFormat("SPACE pause | [ / ] speed | S step | F fast-fwd | H hydrology [%s] | L biota [%s] | W live [%s] | R reseed | +/-",
                     phase3 ? "on" : "off", planet.biotaPopulated() ? "on" : "off", liveWorld ? "on" : "off"));
     line("F5 save | F9 load | F12 screenshot | F2 reload planet.cfg");
@@ -879,6 +917,31 @@ void Viewer::renderFrame() {
                      t == SettleTier::City ? Color{250, 230, 150, 255} : Color{225, 210, 175, 255});
         }
     }
+    // 3D realm labels (with the territory view): name kingdoms/empires at their capital.
+    if (showNationBorders && !planet.nationList().empty()) {
+        Vec3 camPos{cam.position.x, cam.position.y, cam.position.z};
+        Vec3 forward = (Vec3{cam.target.x, cam.target.y, cam.target.z} - camPos).normalized();
+        Vec3 right   = forward.cross(Vec3{cam.up.x, cam.up.y, cam.up.z}).normalized();
+        Vec3 up      = right.cross(forward);
+        double fovRad = cam.fovy * M_PI / 180.0, aspect = (double)view3DW / view3DH;
+        double projH  = std::tan(fovRad * 0.5), projW = projH * aspect;
+        for (const Nation& nat : planet.nationList()) {
+            if (nat.tier == NationTier::CityState) continue;     // declutter: only multi-settlement realms
+            if (nat.capital < 0 || nat.capital >= (int)planet.settlements.size()) continue;
+            int cell = planet.settlements[nat.capital].cell;
+            if (cell < 0 || cell >= (int)planet.cells.size()) continue;
+            int font = nat.tier == NationTier::Empire ? 16 : 14;
+            Vec3 lp = rotateZ(planet.cells[cell].unit, planet.cfg.axialTilt)
+                      * (visBase + (double)planet.cells[cell].elevation * elevExagg + 0.035);
+            if (lp.dot(camPos) <= 0.0) continue;
+            Vec3 rel = lp - camPos; double z = rel.dot(forward); if (z <= 0.0) continue;
+            float sx = (float)((rel.dot(right) / (projW * z) * 0.5 + 0.5) * view3DW);
+            float sy = (float)((0.5 - rel.dot(up) / (projH * z) * 0.5) * view3DH);
+            int w = MeasureText(nat.name.c_str(), font);
+            DrawText(nat.name.c_str(), (int)sx - w / 2 + 1, (int)sy - font - 7, font, Color{0, 0, 0, 205});
+            DrawText(nat.name.c_str(), (int)sx - w / 2,     (int)sy - font - 8, font, Color{245, 235, 210, 255});
+        }
+    }
 
     renderMap2D();
     renderLiveInfo();
diff --git a/src/sim/Planet.cpp b/src/sim/Planet.cpp
index 8245ea6..745ab1a 100644
--- a/src/sim/Planet.cpp
+++ b/src/sim/Planet.cpp
@@ -63,6 +63,7 @@ void Planet::buildGeometry() {
     settlements.clear(); sCivRng = cfg.seed ? (cfg.seed ^ 0x017B1A2Eu) : 0x017B1A2Eu;
     sCellSettlement.assign(cells.size(), -1); sHabitability.clear();
     sCivCond.clear(); sCivDrought.clear();
+    nations.clear(); sCellNation.assign(cells.size(), -1); sSettleNation.clear();
 }
 
 void Planet::clearDerivedState() {
diff --git a/src/sim/Planet.hpp b/src/sim/Planet.hpp
index 5c47368..20a6d51 100644
--- a/src/sim/Planet.hpp
+++ b/src/sim/Planet.hpp
@@ -5,6 +5,7 @@
 #include "PlanetBiota.hpp"   // BiotaKind, Organism, CellBiota
 #include "PlanetGeography.hpp" // FeatureKind, GeoFeature
 #include "PlanetCiv.hpp"       // Settlement, SettleTier, CivUpdate
+#include "PlanetNation.hpp"    // Nation, NationTier
 #include "PlanetEcoregions.hpp" // Ecoregion
 #include <vector>
 #include <memory>
@@ -22,6 +23,7 @@ public:
     std::vector<GeoFeature> geoFeatures; // named geographic features / the atlas (saved v17+)
     std::vector<Ecoregion> ecoRegions; // named ecological provinces (saved v19+)
     std::vector<Settlement> settlements; // civilization: settlements placed once, grow/decline (saved v20+)
+    std::vector<Nation> nations; // realms grouped from settlements (derived each computeTerritory, not saved)
 
     // Phase flag: false during Phase-1 forming (modest, original tectonics that
     // settle), true during Phase-2 drift. Gates the increment-4 orogeny boosts
@@ -187,6 +189,16 @@ public:
     bool     settlementsPlaced() const { return !settlements.empty(); }
     const std::vector<int>&    cellSettlement() const { return sCellSettlement; } // settlement index per cell (-1)
     const std::vector<double>& habitability()   const { return sHabitability; }   // 0..1 per cell (derived)
+
+    // Territory & nations (PlanetNation.cpp). computeTerritory() groups settlements into realms
+    // (capital + vassal towns) and claims cells within each settlement's size-scaled influence range
+    // (wilderness frontiers between realms). Purely derived from the settlement set, so it is recomputed
+    // (on placement / load / each sim year), not saved -- step-back replays it as populations restore.
+    void computeTerritory();
+    bool nationsBuilt() const { return !nations.empty(); }
+    const std::vector<Nation>& nationList()    const { return nations; }
+    const std::vector<int>&    cellNation()    const { return sCellNation; }   // nation index per cell (-1 = wilderness/sea)
+    const std::vector<int>&    settleNation()  const { return sSettleNation; } // nation index per settlement (-1 = dead)
     // Per-settlement live conditions (derived each stepCivilization; not saved). condition = the combined
     // environmental multiplier on carrying capacity (1 = normal, <1 = hardship, >1 = boom); drought =
     // current drought severity 0..1. Parallel to `settlements`. Used by the viewer for tint + events.
@@ -318,6 +330,9 @@ private:
     std::vector<double>        sHabitability;
     std::vector<double>        sCivCond, sCivDrought;   // per-settlement live conditions (derived)
     uint32_t                   sCivRng = 1;
+    // Territory & nations (derived from settlements; not saved). sCellNation: nation index per cell
+    // (-1 = wilderness/ocean); sSettleNation: nation index per settlement.
+    std::vector<int>           sCellNation, sSettleNation;
 
     // Biota: derived density scalars (0..1; recomputed each tick, not saved) and the
     // on-demand discrete population (saved). sHasBiota latches once generated/loaded.
diff --git a/src/sim/PlanetIO.cpp b/src/sim/PlanetIO.cpp
index 95124ed..b6a31df 100644
--- a/src/sim/PlanetIO.cpp
+++ b/src/sim/PlanetIO.cpp
@@ -55,12 +55,13 @@
     D(civSiteVariety) D(civGrowthMin) D(civHarvestVar) D(civDroughtStrength) D(civDroughtPeriod) D(civDroughtThresh) \
     D(civDroughtArid) D(civColdYearStrength) D(civFloodBonus) D(civFamineRate) \
     D(civStormDeathRate) D(civHurricaneDeathMult) \
+    D(civTerritoryBase) D(civTerritoryScale) D(civTerritoryMax) D(civVassalRange) D(civEmpirePop) \
     I(subdivisions) I(plateCount) I(beltWidth) I(splitCheckEvery) I(stalemateWindows) \
     I(miniPlateCells) I(fuseMinPlates) I(babyMinCells) I(seaLevelEvery) \
     I(climateWindPasses) I(climateMoistureSmooth) I(seasonContinentRings) I(weatherSystemMax) \
     I(volcanoMaxCount) \
     I(geoContinentMinCells) I(geoSeaMaxCells) I(geoRangeMinCells) I(geoMaxRivers) I(geoMaxPeaks) \
-    I(geoOceanDeep) I(civMaxSettlements) \
+    I(geoOceanDeep) I(civMaxSettlements) I(civEmpireMinMembers) \
     I(bioFloraSlots) I(bioFaunaSlots) I(bioFungaSlots) \
     I(bioFloraPoints) I(bioFaunaPoints) I(bioFungaPoints) I(bioMarineCoastRings) \
     U(seed)
@@ -279,6 +280,11 @@ std::string validateConfig(const PlanetConfig& cfg) {
     E(rng(cfg.civFamineRate, 0.0, 10.0, "civFamineRate"));
     E(rng(cfg.civStormDeathRate, 0.0, 10.0, "civStormDeathRate"));
     E(rng(cfg.civHurricaneDeathMult, 1.0, 50.0, "civHurricaneDeathMult"));
+    E(rng(cfg.civTerritoryBase, 0.0, 3.14159, "civTerritoryBase"));
+    E(rng(cfg.civTerritoryScale, 0.0, 3.14159, "civTerritoryScale"));
+    E(rng(cfg.civTerritoryMax, 0.01, 3.14159, "civTerritoryMax"));
+    E(rng(cfg.civVassalRange, 0.0, 20.0, "civVassalRange"));
+    E(rng(cfg.civEmpirePop, 1.0, 1.0e12, "civEmpirePop"));
     E(irng(cfg.subdivisions, 0, 7, "subdivisions"));
     E(irng(cfg.plateCount, 1, 100, "plateCount"));
     E(irng(cfg.beltWidth, 1, 12, "beltWidth"));
@@ -300,6 +306,7 @@ std::string validateConfig(const PlanetConfig& cfg) {
     E(irng(cfg.geoMaxPeaks, 0, 100000, "geoMaxPeaks"));
     E(irng(cfg.geoOceanDeep, 1, 1000, "geoOceanDeep"));
     E(irng(cfg.civMaxSettlements, 0, 1000000, "civMaxSettlements"));
+    E(irng(cfg.civEmpireMinMembers, 1, 1000000, "civEmpireMinMembers"));
     E(irng(cfg.bioFloraSlots, 1, 1000, "bioFloraSlots"));
     E(irng(cfg.bioFaunaSlots, 1, 1000, "bioFaunaSlots"));
     E(irng(cfg.bioFungaSlots, 1, 1000, "bioFungaSlots"));
diff --git a/src/sim/PlanetNation.cpp b/src/sim/PlanetNation.cpp
new file mode 100644
index 0000000..a8e2e8b
--- /dev/null
+++ b/src/sim/PlanetNation.cpp
@@ -0,0 +1,101 @@
+#include "Planet.hpp"
+#include "NameGen.hpp"
+#include <algorithm>
+#include <cmath>
+
+// --- Civilization Step 3: territory & nations (realms) -----------------------
+// Group settlements into realms (capital + vassal towns) and claim cells within each settlement's
+// size-scaled influence range, leaving wilderness frontiers. A pure deterministic function of the
+// settlement set (positions + populations) -- no RNG (tectonic stream untouched), recomputed rather
+// than saved, so the live stepper rewinds it for free as populations restore.
+
+const char* nationTierName(NationTier t) {
+    switch (t) {
+        case NationTier::Empire:    return "Empire";
+        case NationTier::Kingdom:   return "Kingdom";
+        case NationTier::CityState: return "City-state";
+    }
+    return "City-state";
+}
+
+void Planet::computeTerritory() {
+    const int n = (int)cells.size();
+    nations.clear();
+    sCellNation.assign(n, -1);
+    sSettleNation.assign(settlements.size(), -1);
+    if (settlements.empty()) return;
+
+    const double sea = cfg.seaLevel, abP = cfg.civAbandonPop;
+    const double seedPop = std::max(1.0, cfg.civSeedPopulation);
+
+    // Influence range each living settlement projects (0 if abandoned). Big cities reach far.
+    std::vector<double> range(settlements.size(), 0.0);
+    for (size_t k = 0; k < settlements.size(); ++k) {
+        if (settlements[k].population < abP || settlements[k].cell < 0 || settlements[k].cell >= n) continue;
+        double rr = cfg.civTerritoryBase + cfg.civTerritoryScale * std::log10(1.0 + settlements[k].population / seedPop);
+        range[k] = std::clamp(rr, 0.0, cfg.civTerritoryMax);
+    }
+    auto ang = [&](int a, int b) {
+        return std::acos(std::clamp(cells[settlements[a].cell].unit.dot(cells[settlements[b].cell].unit), -1.0, 1.0));
+    };
+
+    // Realm grouping: process settlements largest -> smallest; a settlement joins the nearest CAPITAL
+    // (a larger, already-processed settlement) whose annexation reach (civVassalRange x its range)
+    // covers it -> a vassal town; otherwise it founds its own nation -> a capital.
+    std::vector<int> order(settlements.size());
+    for (size_t k = 0; k < order.size(); ++k) order[k] = (int)k;
+    std::sort(order.begin(), order.end(), [&](int a, int b) {
+        if (settlements[a].population != settlements[b].population) return settlements[a].population > settlements[b].population;
+        return a < b;
+    });
+    std::vector<int> capitalOf(settlements.size(), -1);
+    for (int s : order) {
+        if (range[s] <= 0.0) continue;
+        int joinCap = -1; double joinAng = 1e9;
+        for (int c : order) {
+            if (c == s) break;                       // order is descending -> rest are smaller
+            if (capitalOf[c] != c) continue;         // candidate must itself be a capital
+            double d = ang(s, c);
+            if (d < cfg.civVassalRange * range[c] && d < joinAng) { joinAng = d; joinCap = c; }
+        }
+        capitalOf[s] = (joinCap >= 0) ? joinCap : s;
+    }
+
+    // Build nation records from the distinct capitals.
+    std::vector<int> capToNation(settlements.size(), -1);
+    for (int s : order) {
+        if (capitalOf[s] < 0) continue;
+        int cap = capitalOf[s];
+        if (capToNation[cap] < 0) {
+            capToNation[cap] = (int)nations.size();
+            Nation nat; nat.id = (uint32_t)nations.size() + 1; nat.capital = cap;
+            nations.push_back(nat);
+        }
+        int ni = capToNation[cap];
+        sSettleNation[s] = ni;
+        nations[ni].members++;
+        nations[ni].totalPop += settlements[s].population;
+    }
+    for (Nation& nat : nations) {
+        nat.tier = (nat.members >= cfg.civEmpireMinMembers || nat.totalPop >= cfg.civEmpirePop) ? NationTier::Empire
+                 : (nat.members >= 2) ? NationTier::Kingdom : NationTier::CityState;
+        const std::string& capName = settlements[nat.capital].name;
+        nat.name = (nat.tier == NationTier::Empire)  ? capName + " Empire"
+                 : (nat.tier == NationTier::Kingdom) ? "Kingdom of " + capName
+                                                     : capName;            // city-state: bare
+    }
+
+    // Per-cell ownership: each land cell goes to the settlement whose influence reaches furthest there
+    // (range - distance, if > 0); else wilderness. The cell's nation is that settlement's nation.
+    for (int i = 0; i < n; ++i) {
+        if (cells[i].elevation <= sea || cells[i].biome == Biome::Ice) continue;
+        double bestScore = 0.0; int owner = -1;
+        for (size_t k = 0; k < settlements.size(); ++k) {
+            if (range[k] <= 0.0 || sSettleNation[k] < 0) continue;
+            double d = std::acos(std::clamp(cells[i].unit.dot(cells[settlements[k].cell].unit), -1.0, 1.0));
+            double score = range[k] - d;
+            if (score > bestScore) { bestScore = score; owner = (int)k; }
+        }
+        if (owner >= 0) sCellNation[i] = sSettleNation[owner];
+    }
+}
diff --git a/src/sim/PlanetNation.hpp b/src/sim/PlanetNation.hpp
new file mode 100644
index 0000000..7f744e1
--- /dev/null
+++ b/src/sim/PlanetNation.hpp
@@ -0,0 +1,24 @@
+#pragma once
+#include <string>
+#include <vector>
+#include <cstdint>
+
+// Civilization Step 3: territory & nations (realms). Settlements are grouped into nations -- a large
+// city is a capital, nearby smaller settlements its vassal towns (a kingdom); isolated settlements are
+// city-states; the largest realms are empires. Each settlement projects an influence range that scales
+// with its population, claiming surrounding cells (with wilderness frontiers between realms). All of
+// this is a DETERMINISTIC function of the settlement set (positions + populations), so it is recomputed
+// rather than saved -- no save-format change, and the live stepper rewinds it for free.
+
+enum class NationTier : uint8_t { CityState, Kingdom, Empire };
+
+struct Nation {
+    uint32_t    id       = 0;
+    int         capital  = -1;   // settlement index of the realm's capital (its largest city)
+    int         members  = 0;    // number of settlements in the realm
+    double      totalPop = 0.0;  // summed population of its settlements
+    NationTier  tier     = NationTier::CityState;
+    std::string name;            // e.g. "Kingdom of X" / "X Empire" / a city-state's bare name
+};
+
+const char* nationTierName(NationTier t);   // "City-state" / "Kingdom" / "Empire"
diff --git a/src/sim/PlanetTypes.hpp b/src/sim/PlanetTypes.hpp
index ecdc19d..4a3119d 100644
--- a/src/sim/PlanetTypes.hpp
+++ b/src/sim/PlanetTypes.hpp
@@ -413,4 +413,12 @@ struct PlanetConfig {
     double civFamineRate        = 0.15;   // /year accelerated population loss when food < population
     double civStormDeathRate    = 0.50;   // /year population loss for a full-strength storm over a settlement
     double civHurricaneDeathMult= 3.0;    // extra storm death multiplier for a hurricane/typhoon
+    // Territory & nations (PlanetNation.cpp): influence range each settlement projects (size-scaled),
+    // realm grouping (vassals/kingdoms), and the empire threshold. Derived -> recomputed, not saved.
+    double civTerritoryBase     = 0.035;  // rad: base influence range of a seed-size village (~220 km)
+    double civTerritoryScale    = 0.05;   // rad added per log10 of (population / seed) -- big cities reach far
+    double civTerritoryMax      = 0.35;   // rad: cap on a single settlement's reach (~2200 km)
+    double civVassalRange       = 1.5;    // a capital annexes smaller settlements within this x its range
+    int    civEmpireMinMembers  = 5;      // realm of >= this many settlements counts as an Empire
+    double civEmpirePop         = 5.0e6;  // ...or total population >= this counts as an Empire
 };
diff --git a/test_nation.cpp b/test_nation.cpp
new file mode 100644
index 0000000..1f9257e
--- /dev/null
+++ b/test_nation.cpp
@@ -0,0 +1,141 @@
+// Headless test for civilization Step 3 (territory & nations / realms). No display needed.
+//
+//   g++ -std=c++17 -O2 -Isrc/sim test_nation.cpp src/sim/IcoSphere.cpp src/sim/Planet.cpp \
+//       src/sim/PlanetTectonics.cpp src/sim/PlanetDrift.cpp src/sim/PlanetErosion.cpp \
+//       src/sim/PlanetHydrology.cpp src/sim/PlanetBiomes.cpp src/sim/PlanetClimate.cpp \
+//       src/sim/PlanetLive.cpp src/sim/PlanetOcean.cpp src/sim/PlanetWeather.cpp \
+//       src/sim/PlanetVolcano.cpp src/sim/PlanetBiota.cpp src/sim/PlanetFloraGen.cpp \
+//       src/sim/PlanetFaunaGen.cpp src/sim/PlanetFungiGen.cpp src/sim/NameGen.cpp \
+//       src/sim/PlanetGeography.cpp src/sim/PlanetEcoregions.cpp src/sim/PlanetCiv.cpp \
+//       src/sim/PlanetNation.cpp src/sim/PlanetIO.cpp -o /tmp/tn && /tmp/tn
+//
+// Verifies: territory ownership + wilderness; bigger cities own more; realm grouping (kingdom vs
+// city-state); tiers; ocean/ice unowned; determinism + RNG isolation; save->load->recompute parity.
+
+#include "Planet.hpp"
+#include <cstdio>
+#include <cmath>
+#include <algorithm>
+#include <sstream>
+
+static int failures = 0;
+static void check(bool cond, const char* what) {
+    std::printf("  [%s] %s\n", cond ? "PASS" : "FAIL", what);
+    if (!cond) ++failures;
+}
+static void settle(Planet& p, int maxSteps = 800) {
+    int run = 0;
+    for (int s = 0; s < maxSteps; ++s) { double mc = p.step(); if (mc < 2.0) { if (++run >= 3) break; } else run = 0; }
+    p.computeClimate(); p.classifyBiomes();
+}
+static void drift(Planet& p, int iters) {
+    p.drifting = true;
+    for (int k = 0; k < iters; ++k) { double dt = p.cflDtMy(); p.advect(dt); p.step(); p.erode(dt); if (k >= iters/2) p.hydrology(dt*0.2); }
+    p.computeClimate(); p.classifyBiomes();
+}
+static int ownedCells(const Planet& p, int nationIdx) {
+    int c = 0; for (int v : p.cellNation()) if (v == nationIdx) ++c; return c;
+}
+
+int main() {
+    PlanetConfig cfg; cfg.subdivisions = 5; cfg.seed = 4242;
+    Planet p; p.generate(cfg); settle(p); drift(p, 400);
+    const int n = (int)p.cells.size();
+    const double sea = p.cfg.seaLevel, yearH = p.cfg.dayLengthHours * p.cfg.yearLengthDays;
+    p.placeSettlements();
+    // run civilization a while so populations diverge (capitals, towns)
+    double lt = 0.0; for (int yr = 0; yr < 600; ++yr) { lt += 2.0 * yearH; p.stepCivilization(2.0 * yearH, lt); }
+
+    std::printf("Territory: extraction\n");
+    p.computeTerritory();
+    check(!p.nationList().empty(), "computeTerritory produces nations");
+    check((int)p.cellNation().size() == n && (int)p.settleNation().size() == (int)p.settlements.size(), "index arrays sized");
+
+    bool seaUnowned = true, ownedIsLand = true, idxOk = true, settleMatch = true;
+    int ownedLand = 0;
+    for (int i = 0; i < n; ++i) {
+        int ni = p.cellNation()[i];
+        if (ni >= (int)p.nationList().size()) idxOk = false;
+        if (ni >= 0) {
+            ++ownedLand;
+            if (p.cells[i].elevation <= sea || p.cells[i].biome == Biome::Ice) ownedIsLand = false;
+        }
+        if ((p.cells[i].elevation <= sea) && ni >= 0) seaUnowned = false;
+    }
+    // every living settlement's home cell belongs to its own nation
+    for (size_t k = 0; k < p.settlements.size(); ++k) {
+        if (p.settlements[k].population < p.cfg.civAbandonPop) continue;
+        int ni = p.settleNation()[k];
+        if (ni < 0 || p.cellNation()[p.settlements[k].cell] != ni) settleMatch = false;
+    }
+    std::printf("    %d nations, %d owned land cells of %d\n", (int)p.nationList().size(), ownedLand, n);
+    check(idxOk, "per-cell nation indices in range");
+    check(seaUnowned, "ocean cells are unowned (wilderness)");
+    check(ownedIsLand, "owned cells are land + non-ice");
+    check(ownedLand > 0 && ownedLand < n, "some land is owned, some is wilderness (influence-limited)");
+    check(settleMatch, "a living settlement's home cell belongs to its own nation");
+
+    std::printf("Territory: bigger realms control more land\n");
+    {
+        double empCells = 0, csCells = 0; int empN = 0, csN = 0;
+        for (size_t ni = 0; ni < p.nationList().size(); ++ni) {
+            int cells = ownedCells(p, (int)ni);
+            if (p.nationList()[ni].tier == NationTier::Empire)      { empCells += cells; ++empN; }
+            else if (p.nationList()[ni].tier == NationTier::CityState) { csCells += cells; ++csN; }
+        }
+        double empAvg = empN ? empCells / empN : 0.0, csAvg = csN ? csCells / csN : 0.0;
+        std::printf("    empire avg %.1f cells (%d) ;  city-state avg %.1f cells (%d)\n", empAvg, empN, csAvg, csN);
+        check(empN == 0 || csN == 0 || empAvg > csAvg, "empires control more territory on average than city-states");
+    }
+
+    std::printf("Territory: realms (kingdoms / city-states / empires)\n");
+    {
+        int kingdoms = 0, cityStates = 0, empires = 0, multiMember = 0;
+        for (const Nation& nat : p.nationList()) {
+            if (nat.tier == NationTier::Empire) ++empires;
+            else if (nat.tier == NationTier::Kingdom) ++kingdoms;
+            else ++cityStates;
+            if (nat.members > 1) ++multiMember;
+            // a kingdom/empire name references its capital; city-states are bare
+            bool ok = !nat.name.empty();
+            if (nat.tier == NationTier::Kingdom && nat.name.rfind("Kingdom of ", 0) != 0) ok = false;
+            check(ok || nat.tier == NationTier::CityState || nat.tier == NationTier::Empire, "nation has a sensible name");
+        }
+        std::printf("    %d empires, %d kingdoms, %d city-states (%d multi-settlement realms)\n",
+                    empires, kingdoms, cityStates, multiMember);
+        check(multiMember >= 1, "at least one realm groups multiple settlements (a kingdom forms)");
+    }
+
+    std::printf("Territory: determinism\n");
+    Planet q; q.generate(cfg); settle(q); drift(q, 400); q.placeSettlements();
+    double lt2 = 0.0; for (int yr = 0; yr < 600; ++yr) { lt2 += 2.0 * yearH; q.stepCivilization(2.0 * yearH, lt2); }
+    q.computeTerritory();
+    bool same = (q.cellNation() == p.cellNation()) && (q.nationList().size() == p.nationList().size());
+    check(same, "computeTerritory is deterministic");
+
+    std::printf("Territory: RNG isolation from tectonics\n");
+    Planet x; x.generate(cfg); settle(x);
+    Planet y; y.generate(cfg); settle(y);
+    for (int k = 0; k < 40; ++k) {
+        double dx = x.cflDtMy(); x.advect(dx); x.step(); x.erode(dx);
+        double dy = y.cflDtMy(); y.advect(dy); y.step(); y.erode(dy);
+        if (k == 20) { y.placeSettlements(); y.stepCivilization(yearH, yearH); y.computeTerritory(); }
+    }
+    bool terrainSame = true;
+    for (int i = 0; i < n; ++i) if (std::fabs(x.cells[i].elevation - y.cells[i].elevation) > 1e-9) terrainSame = false;
+    check(terrainSame, "computeTerritory never perturbs tectonic evolution");
+
+    std::printf("Territory: save -> load -> recompute parity\n");
+    {
+        std::stringstream ss(std::ios::in | std::ios::out | std::ios::binary);
+        p.writeState(ss);
+        Planet r;
+        bool ok = r.readState(ss, true, true, true, true, true, true, true, true, true, true, true);
+        check(ok, "readState accepts the v20 stream");
+        r.computeTerritory();   // territory is derived, not saved -- recompute on both sides must match
+        check(r.cellNation() == p.cellNation(), "territory recomputed after load matches (derived, not saved)");
+    }
+
+    std::printf(failures ? "\nFAILURES: %d\n" : "\nALL NATION CHECKS PASSED\n", failures);
+    return failures ? 1 : 0;
+}