There are numerous causes and explanations of World War ll. World War II was a brutal total war won by the Allied powers of the US, Great Britain, and the Soviet Union against the Axis powers of Nazi Germany, Italy, and Japan. This war saw mass human killing, especially in the Holocaust, and the first use of an atomic bomb.
Some causes: the impact of the Treaty of Versailles following WWI, the worldwide economic depression, failure of appeasement, the rise of militarism in Germany and Japan, and the failure of the League of Nations.
