The WWI argument isn't off the wall. U.S. involvement tipped the war heavily in the Allies favor, leading to the Treaty of Versailles, where the Germans were forced to agree to an exceedingly punitive arrangement that led to economic collapse and hyper-inflation....paving the way for the rise of a mustachioed maniac. Had the U.S. not entered WWI, they would have slogged along in a stalemate and the war would have ended with more favorable terms for Germany that would have allowed them to prosper post-war and there never would have been a WWII.