The Allies of World War II were the countries that opposed the Axis powers during the Second World War (1939–1945). The Allies became involved in World War II either because they had already been invaded, were directly threatened with invasion by the Axis or because they were concerned that the Axis powers would come to control the world.

The anti-German coalition at the start of the war (1 September 1939) consisted of France, Poland and the United Kingdom, soon to be joined by the British dominions (Australia, Canada, New Zealand, and South Africa). After 1941, the leaders of the United States of America, the British Commonwealth, and the Soviet Union known as the "Big Three", held leadership of the allied powers. China, at that time, was also a major Ally. Other Allies included Belgium, Brazil, Czechoslovakia, Ethiopia, Greece, India (as part of the British Empire), Mexico, the Netherlands, Norway and Canada.

During December 1941, US President Franklin Roosevelt devised the name "United Nations" for the Allies. He referred to the Big Three and China as a "trusteeship of the powerful", and then later the "Four Policemen". The Declaration by United Nations on 1 January 1942 was the basis of the modern United Nations (UN). At the Potsdam Conference of July–August 1945, Roosevelt's successor, Harry S. Truman, proposed that the foreign ministers of China, France, the Soviet Union, the United Kingdom and the United States "should draft the peace treaties and boundary settlements of Europe", which led to the creation of the Council of Foreign Ministers.

World War II, or the Second World War (often abbreviated as WWII or WW2), was a global war that was under way by 1939 and ended in 1945. It involved a vast majority of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis. It was the most widespread war in history, with more than 100 million people serving in military units. In a state of "total war", the major participants placed their entire economic, industrial, and scientific capabilities at the service of the war effort, erasing the distinction between civilian and military resources. Marked by significant events involving the mass death of civilians, including the Holocaust and the only use of nuclear weapons in warfare, it resulted in 50 million to over 70 million fatalities. These deaths make the war the deadliest conflict in human history.