World War II lasted from 1939 to 1945 and involved two opposing military alliances: the Allies and the Axis powers. The war was started by Germany's invasion of Poland in 1939 and ended with the defeat of Nazi Germany and Japan in 1945. Some key events included Hitler's rise to power in Germany and Nazi expansion across Europe in the early years of the war, as well as Japan's attack on Pearl Harbor that drew the United States into the conflict in 1941. After years of fighting, the Allied powers defeated the Axis powers on both the Eastern and Western Fronts in Europe and in the Pacific.