The Declaration by United Nations was the main treaty that formalized the Allies of World War II and was signed by 47 national governments between 1942 and 1945. On New Year's Day 1942, during the Arcadia Conference, the Allied "Big Four"—the United Kingdom, the United States, the Soviet Union, and China—signed a short document which later came to be known as the United Nations Declaration, and the next day the representatives of 22 other nations added their signatures.The other original signatories in the next day (2 January 1942) were the four dominions of the British Commonwealth (Australia, Canada, New Zealand, and South Africa); eight European governments-in-exile (Belgium, Czechoslovakia, Greece, Luxembourg, Netherlands, Norway, Poland, and Yugoslavia); nine countries in The Americas (Costa Rica, Cuba, Dominican Republic, El Salvador, Guatemala, Haiti, Honduras, Nicaragua, and Panama); and one non-independent government, the British-appointed Government of India.

The Declaration by United Nations became the basis of the United Nations (UN), which was formalized in the UN Charter, signed by 50 countries on 26 June 1945.