Despite its preeminent position in the just war tradition, the concept of proportionality is not well understood by military leaders. Especially lacking is a realization that there are four distinct types of proportionality. In determining whether a particular resort to war is just, national leaders must consider the proportionality of the conflict, i.e., balance the expected gain or just redress against the total harm likely to be inflicted by the impending armed action. This proportionality consideration is called jus ad bellum (...) proportionality. The second type of proportionality discussed is a continuing re-evaluation of the proportionality, taking into account the changing situation. The last true proportionality discussed is a consideration of hostile action taking during a conflict, or jus in bello proportionality. This consideration weighs the expected military gain of a particular action against the collateral damage and injuries that are expected from the act. Finally, the fourth category addressed is ‘political proportionality’, which is not a true proportionality consideration, but is rather self-interested deliberation on the possible political and military outcomes of actions. In some instances, it could be referred to as a measured response. A better understanding of proportionality in all its forms will help military leaders to give appropriate and moral advice on the potential resort to war, as well as on the conduct of military actions in war. The recent US war with Iraq in the spring of 2003 in defiance of the wishes of the majority of the UN Security Council, and especially with the potential for high noncombatant casualties in Baghdad, makes a discussion of the proper conduct of war even more poignant. (shrink)