The Bush administration reacted to the horrific September 11th attacks by proclaiming a right to preemptive self-defence, making preemption official US military doctrine. A preemptive war doctrine is, so it argued, the only way to make the United States safe. The Bush administration rightly points to the changed nature of military threats and poses a dilemma for scholars of just war theory: how long, in an era of terrorism and weapons of mass destruction, can states afford to wait to use their military force in self-defence? But the administration's doctrine is actually also a preventive war doctrine. And although the doctrine seems compelling at first glance, the logic of the just war tradition's prohibition on preventive war still holds.

Preemptive military action is undertaken to eliminate an immediate and credible threat of grievous harm. Those acting preemptively believe that an adversary is about to attack, that the assault is inevitable, and that a preemptive strike can eliminate the threat or at least reduce the harm that the anticipated assault would cause. Just war theory and international law grant the legitimacy of self-defence in response to an armed attack, and if preemption is self-defence against imminent assault, it is legitimate. Thus, Michael Walzer argues, ‘states may use military force in the face of threats of war, whenever the failure to do so would seriously risk their territorial integrity or political independence’. Preemption may only be considered justified if one has a justified fear of imminent attack, where the potential attacker has a clear intent to cause injury, is actively preparing to do so, and when waiting until the threat is realised greatly increases the risk.

By contrast, a preventive war is undertaken when a state believes that war with a potential adversary is possible or likely at some future date and that, if it waits, it will lose important military advantages. In this case, the threat is not imminent or even certain to materialise in the near future. Rather, the preventive attacker has made a worst-case scenario their working assumption: their potential adversary will attack if they can at some point, and no negotiation or change in the adversary's goals will intervene to stop the assault. In sum: although preemption, when initiated under specific limited circumstances, is generally considered legal, legitimate, and prudent, preventive war is generally considered illegal, illegitimate and imprudent. Indeed, preventive war is often associated with aggression.