It's a simple point, but an important one, and one that gets overlooked. Whether or not you think a particular war is moral and good, the fact remains that war is illegal. Actual defense by a country when attacked is legal, but that only occurs once another country has actually attacked, and it must not be used as a loophole to excuse wider war that is not employed in actual defense.
Needless to say, a strong moral argument can be made for preferring the rule of law to the law of rulers. If those in power can do anything they like, most of us will not like what they do. Some laws are so unjust that when they are imposed on ordinary people, they should be violated. But allowing those in charge of a government to engage in massive violence and killing in defiance of the law is to sanction all lesser abuses as well, since no greater abuse is imaginable. It's understandable that proponents of war would rather ignore or "re-interpret" the law than properly change the law through the legislative process, but it is not morally defensible.
http://www.liberalohioan.com/Swanson-WarIsALie.html