Answer :
Following WWI, America still followed a foreign policy of isolationism. We didn't want to get involved in world affairs because it wasn't something we had been historically known to do. We followed this policy through the beginning of the twentieth century, until involvement in WWII made us a superpower, and our policies changed from isolationism to interventionism (and briefly containment during the Cold War).
America was the last country to enter the war and against the war. Because they do not want war.
