diff --git a/src/client/controllers/BuildPreviewController.ts b/src/client/controllers/BuildPreviewController.ts
index 416bc2cdb..19a70ab32 100644
--- a/src/client/controllers/BuildPreviewController.ts
+++ b/src/client/controllers/BuildPreviewController.ts
@@ -8,7 +8,10 @@
  */
 
 import { EventBus } from "../../core/EventBus";
-import { wouldNukeBreakAlliance } from "../../core/execution/Util";
+import {
+  listNukeBreakAlliance,
+  wouldNukeBreakAlliance,
+} from "../../core/execution/Util";
 import {
   BuildableUnit,
   PlayerBuildableUnitType,
@@ -38,6 +41,22 @@ export function shouldPreserveGhostAfterBuild(unitType: UnitType): boolean {
   return unitType === UnitType.AtomBomb || unitType === UnitType.HydrogenBomb;
 }
 
+/**
+ * Whether a SAM belongs in the nuke trajectory preview's threat set.
+ * Allied SAMs are excluded unless the strike would betray that ally —
+ * the alliance breaks at launch, so their SAMs will engage the nuke.
+ * (Own SAMs never threaten; the caller filters those out first.)
+ */
+export function samThreatensNukePreview(
+  samOwnerSmallId: number,
+  allySmallIds: ReadonlySet<number>,
+  betrayedSmallIds: ReadonlySet<number>,
+): boolean {
+  return (
+    !allySmallIds.has(samOwnerSmallId) || betrayedSmallIds.has(samOwnerSmallId)
+  );
+}
+
 export class BuildPreviewController implements Controller {
   /** Current ghost (null when no build type is active). */
   private ghostUnit: { buildableUnit: BuildableUnit } | null = null;
@@ -312,15 +331,29 @@ export class BuildPreviewController implements Controller {
     const srcX = this.game.x(bestSilo.tile());
     const srcY = this.game.y(bestSilo.tile());
 
-    // Non-allied SAMs threaten the trajectory; own + allied SAMs don't.
+    // Non-allied SAMs threaten the trajectory; own + allied SAMs don't —
+    // except allies this strike would betray: the alliance breaks at launch
+    // (NukeExecution.maybeBreakAlliances), so their SAMs will intercept.
+    // listNukeBreakAlliance is the same function the sim uses there.
     const allyIds = new Set<number>();
     for (const a of myPlayer.allies()) allyIds.add(a.smallID());
+    const betrayedIds: ReadonlySet<number> =
+      allyIds.size > 0
+        ? listNukeBreakAlliance({
+            game: this.game,
+            targetTile: tileRef,
+            magnitude: this.game.config().nukeMagnitudes(type),
+            threshold: this.game.config().nukeAllianceBreakThreshold(),
+          })
+        : new Set();
     const sams: SAMInfo[] = [];
     for (const s of this.game.units(UnitType.SAMLauncher)) {
       if (!s.isActive()) continue;
       const owner = s.owner();
       if (owner === myPlayer) continue;
-      if (allyIds.has(owner.smallID())) continue;
+      if (!samThreatensNukePreview(owner.smallID(), allyIds, betrayedIds)) {
+        continue;
+      }
       const r = this.game.config().samRange(s.level());
       sams.push({
         x: this.game.x(s.tile()),
diff --git a/tests/client/controllers/BuildPreviewController.test.ts b/tests/client/controllers/BuildPreviewController.test.ts
index 3dafc5469..b0d475b63 100644
--- a/tests/client/controllers/BuildPreviewController.test.ts
+++ b/tests/client/controllers/BuildPreviewController.test.ts
@@ -1,5 +1,8 @@
 import { describe, expect, test } from "vitest";
-import { shouldPreserveGhostAfterBuild } from "../../../src/client/controllers/BuildPreviewController";
+import {
+  samThreatensNukePreview,
+  shouldPreserveGhostAfterBuild,
+} from "../../../src/client/controllers/BuildPreviewController";
 import { UnitType } from "../../../src/core/game/Game";
 
 describe("BuildPreviewController ghost preservation (locked nuke / Enter confirm)", () => {
@@ -30,3 +33,23 @@ describe("BuildPreviewController ghost preservation (locked nuke / Enter confirm
     });
   });
 });
+
+describe("samThreatensNukePreview (nuke trajectory threat set, #4226)", () => {
+  const allies = new Set([2, 3]);
+
+  test("non-allied SAM threatens the trajectory", () => {
+    expect(samThreatensNukePreview(5, allies, new Set())).toBe(true);
+  });
+
+  test("allied SAM does not threaten when the strike breaks no alliance", () => {
+    expect(samThreatensNukePreview(2, allies, new Set())).toBe(false);
+  });
+
+  test("would-be-betrayed ally's SAM threatens (alliance breaks at launch)", () => {
+    expect(samThreatensNukePreview(2, allies, new Set([2]))).toBe(true);
+  });
+
+  test("other allies' SAMs still excluded when a different ally is betrayed", () => {
+    expect(samThreatensNukePreview(3, allies, new Set([2]))).toBe(false);
+  });
+});