So Iraq was not similar to the German or Japanese or the (hypothetical) Rwandan case: the war was not a response to aggression or a humanitarian intervention. Its cause was not (as in 1991) an actual Iraqi attack on a neighboring state or even an imminent threat of attack; nor was it an actual, ongoing massacre. The cause was regime change, directly—which means that the U.S. government was arguing for a significant expansion of the doctrine of jus ad bellum. The existence of an aggressive and murderous regime, it claimed, was a legitimate occasion for war, even if the regime was not actually engaged in aggression or mass murder. In more familiar terms, this was an argument for preventive war, but the reason for the preventive attack wasn’t the standard perception of a dangerous shift in the balance of power that would soon leave “us” helpless against “them.” It was a radically new perception of an evil regime.
No one who has experienced, or reflected on, the politics of the twentieth century can doubt that there are evil regimes. Nor can there be any doubt that we need to design a political/military response to such regimes that recognizes their true character. Even so, I do not believe that regime change, by itself, can be a just cause of war.