1) Attack on Pearl Harbor ( December 7, 1941)
2) Allies falling (with the allies falling, the US were afraid they would have to fight the war by themselves, and so they intervened later on)
3) Germany declaring war (After the US declared war on Japan because of the Pearl Harbor event, Germany and Italy declared war on them (because the Nazis and Fascists were allied with the Japanese)
4) Winston Churchill being able to convince President Roosevelt to join the war effort
5) The war was a simple way to solve the problems presented by the Great Depression, which was to give people jobs
hope this helps