The Allies, formally referred to as the United Nations from 1942, were an international military coalition formed during World War II (1939–1945) to oppose the Axis powers. Its principal members were the "Big Four" – the United Kingdom, Soviet Union, United States, and China.
Membership in the Allies varied during the course of the war. When the conflict broke out on 1 September 1939, the Allied coalition consisted of the United Kingdom, France, and Poland, as well as their respective dependencies, such as British India. They were joined by the independent dominions of the British Commonwealth: Canada, Australia, New Zealand and South Africa. Consequently, the initial alliance resembled that of the First World War. As Axis forces began invading northern Europe and the Balkans, the Allies added the Netherlands, Belgium, Norway, Greece, and Yugoslavia. The Soviet Union, which initially had a nonaggression pact with Germany and participated in its invasion of Poland, joined the Allies after the German invasion of the Soviet Union in June 1941. The United States, while providing some material support to European Allies since September 1940, remained formally neutral until the Japanese attack on Pearl Harbor in December 1941, after which it declared war and officially joined the Allies. China had already been at war with Japan since 1937, and formally joined the Allies in December 1941.
