diff --git a/src/simulator/crates/mc-ai/src/tactical/apply.rs b/src/simulator/crates/mc-ai/src/tactical/apply.rs
index c361c060..4b9646ea 100644
--- a/src/simulator/crates/mc-ai/src/tactical/apply.rs
+++ b/src/simulator/crates/mc-ai/src/tactical/apply.rs
@@ -193,6 +193,24 @@ pub fn apply_tactical_action(state: &mut TacticalState, action: &Action) {
                 u.pending_promotion_choices.clear();
             }
         }
+        Action::DeclareWar { target } => {
+            // Rollout approximation: flip the actor↔target relation to war so
+            // the lookahead's movement/combat immediately treats the target as
+            // an enemy. Mirrors the real dispatch's sender-side war state
+            // (the shared cell flips on delivery/combat in the live engine).
+            let actor = state.current_player as usize;
+            let t = *target as usize;
+            if let Some(p) = state.players.get_mut(actor) {
+                if let Some(slot) = p.relations.get_mut(t) {
+                    *slot = -1;
+                }
+            }
+            if let Some(p) = state.players.get_mut(t) {
+                if let Some(slot) = p.relations.get_mut(actor) {
+                    *slot = -1;
+                }
+            }
+        }
     }
 }
 
diff --git a/src/simulator/crates/mc-ai/src/tactical/diplomacy.rs b/src/simulator/crates/mc-ai/src/tactical/diplomacy.rs
new file mode 100644
index 00000000..9dd1e4fb
--- /dev/null
+++ b/src/simulator/crates/mc-ai/src/tactical/diplomacy.rs
@@ -0,0 +1,191 @@
+//! Strategic war-declaration decisions (p3-16).
+//!
+//! Canonical model is courier-diplomacy (`COMMUNICATIONS.md`
+//! §"War declaration semantics"): every pair starts at PEACE, and war
+//! begins only when a player *dispatches* a war-dec envelope. Nothing made
+//! the AI dispatch one — `decide_tactical_actions` had no diplomacy step and
+//! there was no `DeclareWar` anywhere in `mc-ai` — so AI-vs-AI sat at
+//! perpetual peace, military units never acquired a target, and the clan
+//! `aggression` personality axis never manifested (a warmonger played like a
+//! builder). This step closes that gap: it opens hostilities against a
+//! *discovered* rival once the military balance clears an aggression-scaled
+//! bar, emitting [`Action::DeclareWar`] which the dispatch routes to
+//! `comms_dispatch::dispatch_war_declaration` (sender enters War on dispatch).
+
+use std::time::Instant;
+
+use crate::evaluator::ScoringWeights;
+use crate::mcts::XorShift64;
+
+use super::movement::{count_military, is_at_war};
+use super::{thresholds, Action, TacticalState};
+
+/// Decide whether to open hostilities against any discovered rival.
+///
+/// Pure and deterministic: keyed only on `state` + the player's
+/// `strategic_axes`. The `rng`/`weights` arguments are accepted for
+/// signature symmetry with the other decision steps but unused — there is
+/// no stochastic element to the threshold, so two calls on the same state
+/// return identical war-decs.
+pub(crate) fn decide_diplomacy(
+    state: &TacticalState,
+    _weights: &ScoringWeights,
+    _rng: &mut XorShift64,
+    deadline: Option<Instant>,
+) -> Vec<Action> {
+    if deadline.map_or(false, |d| Instant::now() >= d) {
+        return Vec::new();
+    }
+    let Some(me) = state.players.get(state.current_player as usize) else {
+        return Vec::new();
+    };
+
+    // No army → no credible war. Declaring with nothing to fight with just
+    // invites a counter-attack; let production build up first.
+    let own_mil = count_military(&me.units);
+    if own_mil == 0 {
+        return Vec::new();
+    }
+
+    // Aggression-scaled superiority bar. `dominance_factor` returns ~1.15 for
+    // a warmonger (aggression 9) and ~1.80 for a cautious clan (aggression 1):
+    // warmongers open hostilities near parity, the cautious need a real edge.
+    let required_ratio = thresholds::dominance_factor(&me.strategic_axes);
+
+    let mut out = Vec::new();
+    for other in &state.players {
+        if other.index == me.index || is_at_war(me, other.index) {
+            continue;
+        }
+        // Discovery gate: the fog projection only surfaces visible enemy
+        // units/cities, so a non-empty roster means we have eyes on this
+        // rival. We cannot war-dec a capital we do not know exists, and a
+        // war with no reachable target is wasted carriers.
+        if other.units.is_empty() && other.cities.is_empty() {
+            continue;
+        }
+        let enemy_mil = count_military(&other.units);
+        if (own_mil as f32) >= (enemy_mil as f32) * required_ratio {
+            out.push(Action::DeclareWar { target: other.index });
+        }
+    }
+    out
+}
+
+#[cfg(test)]
+mod tests {
+    use std::collections::BTreeMap;
+
+    use crate::evaluator::ScoringWeights;
+    use crate::mcts::XorShift64;
+    use crate::tactical::{
+        Action, TacticalCity, TacticalMap, TacticalPlayerState, TacticalState, TacticalUnit,
+    };
+
+    fn warrior(id: u32, owner_hex: (i32, i32)) -> TacticalUnit {
+        TacticalUnit {
+            id,
+            kind: "dwarf_warrior".into(),
+            hex: owner_hex,
+            hp: 10,
+            hp_max: 10,
+            moves_left: 2,
+            ..Default::default()
+        }
+    }
+
+    fn player(index: u8, aggression: i32, units: Vec<TacticalUnit>, cities: Vec<TacticalCity>) -> TacticalPlayerState {
+        let mut axes = BTreeMap::new();
+        axes.insert("aggression".to_string(), aggression);
+        TacticalPlayerState {
+            index,
+            units,
+            cities,
+            strategic_axes: axes,
+            // All pairs default to peace (0); courier model start state.
+            relations: vec![0i8; 4],
+            ..Default::default()
+        }
+    }
+
+    fn city(id: u32, hex: (i32, i32)) -> TacticalCity {
+        TacticalCity {
+            id,
+            hex,
+            population: 3,
+            tiles_worked: Vec::new(),
+            production_queue: Vec::new(),
+            buildings: Vec::new(),
+            health: 25,
+            is_capital: true,
+        }
+    }
+
+    fn state_with(players: Vec<TacticalPlayerState>, current: u8) -> TacticalState {
+        TacticalState {
+            current_player: current,
+            turn: 10,
+            map: TacticalMap { width: 4, height: 4, tiles: Vec::new() },
+            players,
+            unit_catalog: Vec::new(),
+            building_catalog: Vec::new(),
+            difficulty_threshold_mult: 1.0,
+        }
+    }
+
+    fn run(state: &TacticalState) -> Vec<Action> {
+        super::decide_diplomacy(state, &ScoringWeights::default(), &mut XorShift64::new(1), None)
+    }
+
+    #[test]
+    fn declares_on_discovered_weaker_rival() {
+        let me = player(0, 5, vec![warrior(1, (0, 0)), warrior(2, (1, 0))], vec![city(0, (0, 0))]);
+        // Rival visible (one unit + a city) but weaker (no military).
+        let them = player(1, 5, vec![], vec![city(1, (9, 9))]);
+        let st = state_with(vec![me, them], 0);
+        let actions = run(&st);
+        assert_eq!(actions.len(), 1, "exactly one war-dec expected");
+        assert!(matches!(actions[0], Action::DeclareWar { target: 1 }));
+    }
+
+    #[test]
+    fn does_not_declare_on_undiscovered_rival() {
+        let me = player(0, 9, vec![warrior(1, (0, 0)), warrior(2, (1, 0))], vec![city(0, (0, 0))]);
+        // Rival has no visible units/cities → not yet discovered.
+        let them = player(1, 5, vec![], vec![]);
+        let st = state_with(vec![me, them], 0);
+        assert!(run(&st).is_empty());
+    }
+
+    #[test]
+    fn does_not_declare_when_already_at_war() {
+        let mut me = player(0, 9, vec![warrior(1, (0, 0))], vec![city(0, (0, 0))]);
+        me.relations[1] = -1; // already at war with slot 1
+        let them = player(1, 5, vec![], vec![city(1, (9, 9))]);
+        let st = state_with(vec![me, them], 0);
+        assert!(run(&st).is_empty());
+    }
+
+    #[test]
+    fn cautious_clan_holds_at_parity_warmonger_strikes() {
+        // Both sides field one military unit (parity).
+        let make = |aggr| {
+            let me = player(0, aggr, vec![warrior(1, (0, 0))], vec![city(0, (0, 0))]);
+            let them = player(1, 5, vec![warrior(9, (9, 9))], vec![city(1, (9, 9))]);
+            state_with(vec![me, them], 0)
+        };
+        // Warmonger (aggression 9, factor ~1.15) — 1 >= 1*1.15 is false, so
+        // even a warmonger needs a slight edge; at strict parity it holds.
+        // Cautious (aggression 1, factor ~1.80) certainly holds.
+        assert!(run(&make(1)).is_empty(), "cautious clan must not declare at parity");
+        assert!(run(&make(9)).is_empty(), "neither declares at strict parity (ratio bar)");
+    }
+
+    #[test]
+    fn no_army_never_declares() {
+        let me = player(0, 9, vec![], vec![city(0, (0, 0))]);
+        let them = player(1, 5, vec![], vec![city(1, (9, 9))]);
+        let st = state_with(vec![me, them], 0);
+        assert!(run(&st).is_empty());
+    }
+}
diff --git a/src/simulator/crates/mc-ai/src/tactical/mod.rs b/src/simulator/crates/mc-ai/src/tactical/mod.rs
index 7f82303b..ceef4a6b 100644
--- a/src/simulator/crates/mc-ai/src/tactical/mod.rs
+++ b/src/simulator/crates/mc-ai/src/tactical/mod.rs
@@ -34,6 +34,7 @@
 pub mod apply;
 pub(crate) mod citizen;
 pub mod combat_predict;
+pub(crate) mod diplomacy;
 pub mod culture_pick;
 pub mod memory;
 pub(crate) mod movement;
@@ -204,6 +205,15 @@ pub enum Action {
         /// `public/resources/promotions/promotions.json`).
         promotion_id: String,
     },
+    /// Declare war on another player's slot via the courier system (p3-16).
+    /// The sender enters `War` the instant the war-dec envelope is
+    /// dispatched (COMMUNICATIONS.md §"War declaration semantics"), which
+    /// is what lets the army then drive on the target. Emitted by
+    /// [`diplomacy::decide_diplomacy`].
+    DeclareWar {
+        /// Target player slot.
+        target: u8,
+    },
 }
 
 /// Compute the full set of tactical actions for the player whose turn it
@@ -224,11 +234,12 @@ pub enum Action {
 /// env var is unset). See p1-22.
 ///
 /// Stable submodule order:
-/// 1. [`movement::decide_movement`]
-/// 2. [`combat_predict::decide_combat`]
-/// 3. [`settle::decide_settle`]
-/// 4. [`production::decide_production`]
-/// 5. [`citizen::decide_citizens`]
+/// 1. [`diplomacy::decide_diplomacy`]
+/// 2. [`movement::decide_movement`]
+/// 3. [`combat_predict::decide_combat`]
+/// 4. [`settle::decide_settle`]
+/// 5. [`production::decide_production`]
+/// 6. [`citizen::decide_citizens`]
 pub fn decide_tactical_actions(
     state: &TacticalState,
     weights: &ScoringWeights,
@@ -241,6 +252,14 @@ pub fn decide_tactical_actions(
     };
 
     let mut actions = Vec::new();
+    // Strategic war-declaration runs first so a war-dec dispatched this turn
+    // is applied before the unit actions that follow (p3-16). The army's
+    // drive on the new enemy materialises next turn, once the relation flip
+    // is reflected in the projection.
+    actions.extend(diplomacy::decide_diplomacy(state, weights, rng, deadline));
+    if is_expired(&deadline) {
+        return actions;
+    }
     actions.extend(movement::decide_movement(state, weights, rng, deadline, memory));
     if is_expired(&deadline) {
         return actions;
diff --git a/src/simulator/crates/mc-ai/src/tactical/movement.rs b/src/simulator/crates/mc-ai/src/tactical/movement.rs
index 92682903..3a818f21 100644
--- a/src/simulator/crates/mc-ai/src/tactical/movement.rs
+++ b/src/simulator/crates/mc-ai/src/tactical/movement.rs
@@ -363,10 +363,14 @@ fn is_military(unit: &TacticalUnit) -> bool {
 
 // ── Enemy / diplomacy enumeration ────────────────────────────────────────
 
-fn is_at_war(me: &TacticalPlayerState, opponent_index: u8) -> bool {
-    // Mirrors `simple_heuristic_ai.gd::_is_at_war` (line 322) — default to
-    // war when a relation slot is missing so the attack gate stays open in
-    // fresh games where the diplomacy table has not been initialized.
+pub(super) fn is_at_war(me: &TacticalPlayerState, opponent_index: u8) -> bool {
+    // Canonical model is courier-diplomacy: pairs start at PEACE and war
+    // begins when a player dispatches a war-dec envelope (COMMUNICATIONS.md
+    // §"War declaration semantics"; p1-01's "missing → war" is superseded,
+    // see p3-16). `project_tactical_relations` fills the relations vec, so a
+    // slot is normally present; the `map_or(true, …)` fallback only fires for
+    // a genuinely absent slot and is left open so a not-yet-projected pair
+    // never silently blocks retaliation.
     if (opponent_index as usize) == (me.index as usize) {
         return false;
     }
@@ -411,7 +415,7 @@ fn collect_enemy_city_positions(
     out
 }
 
-fn count_military(units: &[TacticalUnit]) -> usize {
+pub(super) fn count_military(units: &[TacticalUnit]) -> usize {
     units.iter().filter(|u| is_military(u)).count()
 }
 
diff --git a/src/simulator/crates/mc-player-api/src/dispatch.rs b/src/simulator/crates/mc-player-api/src/dispatch.rs
index fb81f68b..064ca7e7 100644
--- a/src/simulator/crates/mc-player-api/src/dispatch.rs
+++ b/src/simulator/crates/mc-player-api/src/dispatch.rs
@@ -1275,6 +1275,13 @@ pub fn apply_ai_action(
             });
             apply_action(state, player, &pa)
         }
+        AiAction::DeclareWar { target } => {
+            // p3-16: route the AI's war-dec through the same courier dispatch
+            // the human uses (PlayerAction::DeclareWar → apply_declare_war →
+            // comms_dispatch::dispatch_war_declaration).
+            let pa = PlayerAction::DeclareWar { on: target };
+            apply_action(state, player, &pa)
+        }
         // Variants without a corresponding PlayerAction wire variant.
         // Quietly no-op so a single unmatched action does not abort the
         // rest of the AI turn. TRACKED: extend the PlayerAction surface
@@ -1378,6 +1385,7 @@ fn ai_action_to_player_action(
                 promotion_id: promotion_id.clone(),
             }))
         }
+        AiAction::DeclareWar { target } => Some(PlayerAction::DeclareWar { on: *target }),
         // No PlayerAction analogue — silent no-ops in apply_ai_action.
         AiAction::AssignCitizen { .. }
         | AiAction::DeploySiege { .. }
@@ -3056,6 +3064,19 @@ mod tests {
         }
     }
 
+    #[test]
+    fn ai_declare_war_maps_to_player_declare_war() {
+        // p3-16: the AI's war-dec must round-trip through the suggest/wire
+        // converter to PlayerAction::DeclareWar so it routes to the same
+        // courier dispatch the human uses.
+        let state = make_state_with_units(vec![(0, 1, 0, 0), (1, 2, 5, 5)]);
+        let action = mc_ai::tactical::Action::DeclareWar { target: 1 };
+        match ai_action_to_player_action(&state, 0, &action) {
+            Some(PlayerAction::DeclareWar { on }) => assert_eq!(on, 1),
+            other => panic!("expected DeclareWar{{on:1}}, got {other:?}"),
+        }
+    }
+
     #[test]
     fn ai_siege_variants_are_silent_no_ops() {
         // DeploySiege / PackSiege / Bombard have no PlayerAction wire