This essay looks at the contemporary just war theory literature on preventive war that has emerged largely in reaction to the US invasion of Iraq. Recent sanctions on Iran and the debate over its nuclear program now suggest the usefulness of a forward looking perspective on preventive strikes, rather than the retroactive analyses offered thus far primarily with reference to Iraq. With Iran closely in mind, I address the various arguments for and against preventive war indicating throughout that the various principled objections to early military action can be overcome in this case. Many of the crucial concrete questions regarding costs and benefits need to be settled in practice, rather than in the realm of political theory. Ultimately, the discussion suggests that Iran is a legitimate candidate for early military action aimed to prevent it from developing nuclear weapons. I argue that in principle, subject to credible intelligence information and requirements of proportionality, a unilateral Israeli strike against Iran will be justifiable, both morally and legally, as self-defense.