Generally speaking, just war theory (JWT) holds that there are two just causes for war: self-defence and ‘other-defence’, the most common type of which is popularly known as ‘humanitarian intervention’. There is however some debate as to whether these serve equally as just causes for preventive war. Whilst this debate is ongoing, those theorists who claim to subscribe to JWT tend to be unified in treating preventive war with a healthy dose of suspicion. Those who oppose preventive war tend to do so on the basis that it fails to fit popular criteria for jus ad bello; particularly, the just cause and last resort criteria. Francisco di Vitoria held that the only just cause for war was “a wrong received”, undermining any justification for preventive war. However, more recent developments in military practice suggest a questioning of an outright ban; most notably, the US-led invasion of Iraq in 2003.

Interestingly, both supporters and critics of preventive war tend to justify their view through the broader logic of JWT; viz., a conception of what is good for both political communities and individuals, and defending those goods. Supporters point to situations where there is ‘grave and imminent risk’ that a nation will suffer aggression as good reasons to resist the oncoming aggression, whilst critics may argue that to attack another political community on the basis of crimes not yet performed is a breach of the same rights that JWT strives to defend.

The advocate of preventive war does capture a point which should be of very real concern to us today. Situations such as the ongoing tensions between Iran and The United States and her allies are situations where – if the rhetoric is to be believed – an ongoing threat to security and liberty is tolerated, and our being subject to aggression is risked in the interest of the rights of the antagonistic, but not yet aggressive, state. Is such tolerance and risk consistent with the logic of just war? When is the risk of being attacked too great to abstain from declaring war in anticipation?

In this paper I will highlight some of the theoretical and practical difficulties in determining what counts as a grave and imminent threat; focussing especially on the complicating case of ‘imminence’ in the face of so-called ‘Weapons of Mass Destruction’. Secondly, I will argue that not only is the notion of preventive war both inconsistent with the defence of the rights of political communities that JWT defends, but is also forbidden by the proportionality requirement of jus ad bellum. The risk of being subject to aggression is the cost of global peace; and whilst political communities can do a great deal to prevent aggression and be prepared in case it occurs, the demands of just war require that this prevention and preparation stop short of declaring war. We must accept a degree of risk in this area.


Nuclear War, Preventive War, Just War Theory, pre-emptive war, Michael Walzer, Jeff McMahan, Ethics, Aggression, Risk, Liberty, Justice


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