World War II began in 1939 when Adolf Hitler invaded Poland. In September 1940 the Axis Powers, Germany, Italy, and Japan, formed. The Allied Powers, Great Britain, the USSR, and the US, formed as a result of needing to combat the growing threat of the Axis Powers. World War II was caused by: 1) Failures of the League of Nations to maintain international peace, 2) A weakened China due to civil war, which allowed Japan to invade and take over,3) Unresolved anger from World War I and the Treaty of Versailles, 4) The Japanese attack on Pearl Harbor on December 7, 1941.