As Jim argues, preventive wars don’t fit neatly into the traditional understanding of just wars. The complication, though, that right-wing preventative war supporters would bring up is that the traditional understanding of wars doesn’t necessarily apply to today’s conflicts. 

Specifically, in the times when just war theory was developed, wars were generally fought by nations that declared war on each other and sent conventional armies into the field against declared combatants. Today the combatants are “terrorists” and “insurgents” who aren’t necessarily associated with a nation and don’t have a unified command. Also, they don’t have a centralized location and could launch a devastating stealth attack anywhere — so we “fight them over there so we don’t have to fight them here.” 

Clearly the concept of a just war has to be updated to accommodate the reality of terrorism. For instance, it seems to be settled that the invasion of Afghanistan was justified under a modern just war theory, even though the nation of Afghanistan itself posed almost no threat to the U.S., because that’s where the terrorists were. 

But stretching just war theory to fit today’s warfare is not easy. The evidence that it’s been stretched too far, as Jim mentions, is that, in hindsight at least, our ability to speculate about the threats posed by terrorists and their sponsors is limited. And, when an updated just war theory is invoked to justify tactics that were deplorable under the old just war definitions, it should be cause for concern.