Few subjects are as potentially important for the study of international political theory in the twenty-first century as the ethical implications of the notion of preemptive war, and the putative distinction between preemption and prevention, and yet, in the post-9/11 environment, few subjects are as difficult to approach in a calm, rational manner. The reason for this strange state of affairs is, I suggest, clear. In most writers’ minds preemption is associated with two reference points; the US National Security Strategy of 2002 (hereafter NSS 2002), which appeared to legitimate a very strong, and perhaps indefensible, doctrine of preemption, and the Iraq War of 2003, which was partly justified in terms of preempting a threat from weapons of mass destruction (WMD) which, in the event, proved not to exist, or at least not there and then.1 Because of these two commonly understood reference points, the issue of preemption has become indelibly associated with the foreign policy of the Bush Administration, and the assumption is that to take any attitude to preemption which does not simply involve condemning the notion out of hand is morally equivalent to endorsing that foreign policy – a position which most writers on international ethics are very unwilling to adopt. I share this latter unwillingness, but it seems to me to be a mistake to take this to mean that the issue of preemption is, in some way, out of bounds. Indiscriminate attacks designed to kill Western civilians pre-date 9/11 and the Bush Administration (and thus were not motivated by Bush’s foreign policy) and there is every reason to think they will continue after 2008 when Bush leaves office.2 One of the possible responses to such attacks may involve the preemptive use of force against the terrorists and those who give them sanctuary, hence the need to develop an ethically acceptable approach to preemption, which is the aim of this essay.3