World War II, also called the Second World War, was a global conflict fought from 1 September 1939 to 2 September 1945 between two major coalitions: the Allies and the Axis. Nearly all countries in the world took part in some way, and more than 100 million people served as combatants. It was the deadliest conflict in history, with an estimated 60 to 75 million deaths, including large numbers of civilians killed by massacre, starvation, disease, bombing, and genocide.
The war grew out of unresolved tensions after World War I and the rise of fascist and militarist regimes in Europe and Asia. Key prewar developments included Japan's invasion of Manchuria, the Spanish Civil War, the Second Sino-Japanese War, and Germany’s annexations of Austria and the Sudetenland. The conflict is generally dated from 1 September 1939, when Nazi Germany invaded Poland; the United Kingdom and France soon declared war, while the Soviet Union also occupied eastern Poland. In 1940, Germany conquered much of western and northern Europe, and after the Battle of Britain the war spread across the Mediterranean, North Africa, the Balkans, and the Atlantic. Germany invaded the Soviet Union in June 1941, opening the Eastern Front (World War II). The war became truly worldwide after Japan attacked Pearl Harbor in December 1941, bringing the United States into the conflict.
Major turning points included the Battle of Midway, Battle of Stalingrad, the Allied landings in Normandy, and the Soviet advance into Central Europe. Germany surrendered on 8 May 1945, and Japan surrendered after the atomic bombings of Hiroshima and Nagasaki and the Soviet invasion of Manchuria, formally ending the war on 2 September 1945. The war led to the occupation of Germany and Japan, the creation of the United Nations, the rise of the United States and Soviet Union as superpowers, the beginning of the Cold War, and the acceleration of decolonization in Asia and Africa.