World War I. The US didn't HAVE to get involved. Of course, neither did Russia, Germany, France, UK....

But let's not forget the Mexican War or the Spanish-American War. The first was a land grab and the second a Hearst-supplied conflict.

The US had justification, as Kendall points out, for the War of 1812. It also had justification for the war against the Barbary pirates ("...to the shores of Tripoli...").