I find it remarkably odd that the US wars you cite as somewhat justifiable are the War of 1812 and the Mexican-American War. WWII was about lots of things, the least of which was to stop a holocaust- our sitting idly by while Hitler murdered undesirables, while Rwanda happened, while Serbs massacred Bosnians, if stepping in to stop these from happening is wrong, then I am truly sorry for you.
"the good man should be both a strong and a brave man; that is, he should be able to fight, he should be able to serve his country as a soldier, if the need arises. There are well-meaning philosophers who declaim against the unrighteousness of war. They are right only if they lay all their emphasis upon the unrighteousness. War is a dreadful thing, and unjust war is a crime against humanity. But it is such a crime because it is unjust, not because it is a war. The choice must ever be in favor of righteousness, and this is whether the alternative be peace or whether the alternative be war. The question must not be merely, Is there to be peace or war? The question must be, Is it right to prevail? Are the great laws of righteousness once more to be fulfilled? And the answer from a strong and virile people must be "Yes," whatever the cost. Every honorable effort should always be made to avoid war, just as every honorable effort should always be made by the individual in private life to keep out of a brawl, to keep out of trouble; but no self-respecting individual, no self-respecting nation, can or ought to submit to wrong." Theodore Roosevelt