Prior to WWI and WWII, the U.S. was overwhelmingly isolationist. The only reason we got involved in WWI was the German telegram intercept to Mexico to attack the U.S. giving Mexico the territory seceded after the Mexican-American war.
The Zimmerman Note was an excuse, TPTB wanted us in that war from the beginning, and finally figured out how to use propaganda to get the rest of the country onboard.