World War II began in Europe in September 1939 when Germany invaded Poland, and in Asia in 1937 when Japan invaded China. Key events included Germany and Italy forming the Axis alliance with Japan in 1940, Japan's attack on Pearl Harbor in 1941 which brought the US into the war as an ally of Britain and the Soviet Union against the Axis powers, the successful Allied invasion of Normandy in 1944, and the US dropping atomic bombs on Hiroshima and Nagasaki in 1945, leading Japan to surrender and end World War II.