World War II began in 1939 after Germany invaded Poland in violation of the Hitler-Stalin pact that had divided Poland between the two countries. The war involved the Axis powers of Germany, Japan, and Italy against the Allied powers led by Britain, France, and later the Soviet Union and the United States. Key events leading to the war included Hitler's expansionist policies of Lebensraum and Anschluss, Britain and France's failed appeasement policy toward Germany, and tensions left over from World War I.