Make NPCs react to being attacked by other actors (Fixes #1588)

apps/openmw/mwclass/npc.cpp

@@ -680,6 +680,14 @@ namespace MWClass
                 !MWBase::Environment::get().getMechanicsManager()->isAggressive(ptr, attacker))
             MWBase::Environment::get().getMechanicsManager()->commitCrime(attacker, ptr, MWBase::MechanicsManager::OT_Assault);
 
+        if (!attacker.isEmpty() && attacker.getClass().getCreatureStats(attacker).getAiSequence().isInCombat(ptr)
+                && !ptr.getClass().getCreatureStats(ptr).getAiSequence().isInCombat(attacker))
+        {
+            // Attacker is in combat with us, but we are not in combat with the attacker yet. Time to fight back.
+            // Note: accidental or collateral damage attacks are ignored.
+            MWBase::Environment::get().getMechanicsManager()->startCombat(ptr, attacker);
+        }
+
         bool wasDead = getCreatureStats(ptr).isDead();
 
         getCreatureStats(ptr).setAttacked(true);