World War II lasted from 1939 to 1945, which is more than seven years. This war resulted in close to 50 million deaths, making it the bloodiest conflict in world history. The two main groups fighting were the Axis powers and the Allies.
The Axis included Germany, Italy and Japan. The Allies consisted of France, Great Britain, the United States, the Soviet Union and China. The Allies emerged as the victors of the war.
The United States entered the war following the Japanese attacks on its military base, Pearl Harbor, in Hawaii on Dec. 7, 1941. The attack prompted the United States to declare war against Japan, resulting in Germany's declaration of war against the United States due to its allied agreement with Japan.