Although not all were unjust wars. Not counting the Indian Wars, the worst was probably the Mexican War, where we invaded another country for no good reason (other than to support a gringo insurgency in Texas) and after defeating them, forced them to give up half their territory. Even many American heroes who fought in that war (many went on to become celebrated Civil War commanders for the Union and Confederacy), did their duty in battle but expressed distate for the way our nation dishonored itself.

The Spanish American War was a trumped-up excuse to build up an overseas empire like the Europeans had, although to be fair, the government was pressured into it by the media. Hawaii was conquered with a bloodless coup instigated by American sugar farmers, and of course, there were all sorts of little excursions and police actions designed to secure American property interests and support local thugs who did business with us.

Having said that, the US has a better record in this regard than most European countries did over the same time period, probably because of our tradition of non-involvement dating back to George Washington, and the constitutional provision that war should be declared by
Congress, not the executive branch (a lot of good THAT has done us lately!). Unfortunately, formal declarations of war seem to have passed on as quaint Victorian custom. The last one I can think of was when Stalin declared war on Japan in 1945.