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 1 January 1942, during the Arcadia Conference in Washington D.C., the Allied "Big Four"—the United States, the United Kingdom, 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 on 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.

The Allies of World War II first expressed their principles and vision for the post-World War II world in the Declaration of St. James's Palace agreed at the First Inter-Allied Conference in June 1941. The Anglo-Soviet Agreement was signed in July 1941 forming a military alliance between the United Kingdom and the Soviet Union. The two main principles of these agreements, a commitment to the war and renunciation of a separate peace, formed the basis for the later Declaration by United Nations.

The Atlantic Charter was agreed a month later between Britain and the United States, to which the other Allies, now including the Soviet Union, agreed to adhere at the Second Inter-Allied Conference in September.

The Declaration by United Nations was drafted during the Arcadia Conference at the White House on December 29, 1941, by US President Franklin D. Roosevelt, British Prime Minister Winston Churchill, and Roosevelt's aide Harry Hopkins. It incorporated Soviet suggestions but left no role for France, which was under German occupation at the time.

Roosevelt coined the term "United Nations" to describe the Allied countries and suggested it as an alternative to the name "Associated Powers" (the U.S. was never formally a member of the Allies of World War I but entered the war in 1917 as a self-styled "Associated Power"). Churchill accepted it and noted that the phrase was used by Lord Byron in the poem Childe Harold's Pilgrimage (Stanza 35).

The parties pledged to uphold the Atlantic Charter, to employ all their resources in the war against the Axis powers, and not to seek to negotiate a separate peace with any of them; similarly, the Allied nations had agreed not to negotiate a separate peace with the Central Powers in World War I.