The laws of war forbid states to use force against each other except in self-defense or with the authorization of the United Nations Security Council. Self-defense is usually understood to mean self-defense against an imminent threat. We model the decision of states to use force against "rogue" states, and argue that under certain conditions it may be proper to expand the self-defense exception to preemptive self-defense. We also consider related issues such as humanitarian intervention, collective security, and the role of the Security Council.