The U.S enters the war- On April 6, 1917, The U.S Congress authorizes a declaration of war against Germany. The United States enters World War I on the side of France and Britain. Wilson cited Germany's violation of its pledge to suspend unrestricted submarine warfare in the North Atlantic and the Mediterranean, as well as its attempts to entice Mexico i\ into an alliance against the U.S, as his reasons for declaring war. (apr 6, 1917