A war (1939–45) in which the Axis Powers (Germany, Italy, and Japan) were defeated by an alliance eventually including the United Kingdom and its dominions, the Soviet Union, and the United States.
Hitler's invasion of Poland in September 1939 led Great Britain and France to declare war on Germany. Germany defeated and occupied France the following year and soon overran much of Europe. Italy joined the war in 1940, and the US and Japan entered following the Japanese attack on the US fleet at Pearl Harbor. Italy surrendered in 1943 and the Allies launched a full-scale invasion in Normandy in June 1944. The war in Europe ended when Germany surrendered in May 1945; Japan surrendered after the US dropped atom bombs on Hiroshima and Nagasaki in August 1945. An estimated 55 million people were killed during the war, including a much higher proportion of civilians than in the First World War