World War II, also known as the Second World War, was a global conflict that lasted from 1939 to 1945. It involved many of the world's nations, including the major powers of the time, and resulted in the deaths of millions of people. The war was fought between the Allies, led by the United States, the Soviet Union, and the United Kingdom, and the Axis powers, led by Germany, Italy, and Japan. The war ended with the defeat of the Axis powers and the establishment of the United Nations to promote international cooperation and prevent future conflicts.