Before the United States Was in World War II By Javan1223 Nov 11, 1918 World War I Ends Jan 30, 1933 Hitler Appointed German Leader Sep 1, 1939 World War II Started Sep 17, 1939 Soviet Union Invades Poland Sep 27, 1939 Warsaw Captured Dec 13, 1939 United Kingdom Wins First Sea War Apr 8, 1940 Norway Invaded May 10, 1940 Winston Churchill Becomes Prime Minister Period: May 26, 1940 to Jun 4, 1940 Miracle of Dunkirk Jun 14, 1940 Paris Falls to Nazis Period: Jul 10, 1940 to Oct 31, 1940 Britain Fights for Its Life Dec 7, 1941 United States Enters World War II This was caused by Japan's attack on Pearl Harbor.