The use of force in self-defence is lawful only if an armed attack occurs – not if one is likely to occur – against a state. Customary international law permits pre-emptive self-defence only when a threat is so grave and imminent that the victim cannot wait to act in self-defence until the attack has actually started. The Bush doctrine of pre-emption as formulated in the National Security Strategy NSS goes beyond this narrow principle and reserves the right to attack pre-emptively even without a definite and imminent threat. The war in Iraq constitutes the first test case in the implementation of the doctrine and calls for an extensive examination of the reasons offered to justify the pre-emptive use of force. This study concludes that the gravity and immediacy of the threat Iraq posed to international security has clearly been exaggerated. The war was launched on the basis of subjective threat assessments; and the high standard of proof that lawful pre-emption demands, was not met. Yet, the Bush doctrine has repercussions beyond Iraq; it is not entirely clear whether pre-emptive force will also be used against other states that the Bush administration finds threatening. The implications of the new doctrine are both uncertain and dangerous. On the basis of its potential for abuse, the right to use force pre-emptively, unnecessarily endangers the already fragile international legal order.