World War II

World War II (1939–1945) was the deadliest conflict in human history, killing an estimated 70–85 million people — roughly 3% of the 1940 world population. It was fought across every ocean and on every continent except Antarctica, involving virtually every nation on Earth in a struggle between the Axis powers (Germany, Italy, Japan) and the Allied nations (led by Britain, the Soviet Union, the United States, and China).

The war began with Germany's invasion of Poland on September 1, 1939, though its roots lay in the unresolved tensions of World War I, the Great Depression, and the aggressive expansionism of fascist and militarist regimes. Hitler's blitzkrieg conquered most of Western Europe by mid-1940. The invasion of the Soviet Union in June 1941 and Japan's attack on Pearl Harbor in December brought the world's two future superpowers into the conflict.

The turning points came in 1942–1943: Stalingrad, El Alamein, Midway, and Guadalcanal halted Axis advances. D-Day (June 6, 1944) opened a second front in Western Europe. Germany surrendered in May 1945; Japan surrendered in August after the atomic bombings of Hiroshima and Nagasaki. The war ended European colonial dominance, created the United Nations, launched the Cold War between the US and USSR, and — through the horror of the Holocaust — established the principle that crimes against humanity demanded international accountability.