World War II, a global conflict fought between 1939 and 1945, involved the vast majority of the world's countries, forming two opposing military alliances: the Allies and the Axis. It began in Europe when Germany invaded Poland on September 1, 1939, prompting declarations of war by France and the United Kingdom. The conflict drew in nations from across the globe, with many mobilizing their resources in pursuit of total war. The main combatants were the Axis powers, led by Germany, Italy, and Japan, against the Allied powers, which included Great Britain, France, the Soviet Union, the United States, and China.
Considered the deadliest conflict in history, World War II resulted in an estimated 40 to 60 million deaths, mostly civilians. The war saw the use of new technologies, such as tanks and aircraft, enabling strategic bombing of cities and the deployment of the first and only nuclear weapons in war. The war in Europe concluded with Germany's surrender on May 8, 1945, while the conflict in Asia ended on September 2, 1945, with Japan's formal surrender. In the aftermath, the United Nations was established to foster international cooperation and prevent future wars.