Form alliances or betray them. Seven diplomatic states (Allied, Non-aggression treaty, Ceasefire, Neutral, War, Grace, Tributary) decide who can shoot at whom, and the server enforces every treaty.
Every relation between two parties is in exactly one of those seven states, and the server decides whether your artillery is allowed to fire. Allied, Non-aggression treaty and Tributary block attacks and territory capture. Ceasefire warns before it cancels an attack. Neutral allows attacks but blocks capture, and War allows both. Grace is a cooldown nobody can skip. Breaking a treaty affects your whole alliance.