World War II: A joint German–Soviet military parade in Brest-Litovsk is held to celebrate the successful invasion of Poland.

World War II, or the Second World War, was a global conflict between two coalitions: the Allies and the Axis powers. Nearly all of the world's countries participated. Tanks and aircraft played major roles, the latter enabling the strategic bombing of cities and delivery of the only nuclear weapons used in war. World War II was the deadliest conflict in history, causing the death of 60 to 75 million people. Millions died as a result of massacres, starvation, disease, and genocides, including the Holocaust. After the Allied victory, Germany, Austria, Japan, and Korea were occupied, and German and Japanese leaders were tried for war crimes.