Allies of World War II

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, United States, Soviet Union, 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 materiel 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.

The "Big Three"—the United Kingdom, the Soviet Union, and the United States—were the principal contributors of manpower, resources and strategy, each playing a key role in achieving victory. Relations between the United Kingdom and the United States were especially close, with their bilateral Atlantic Charter forming the basis of their alliance. A series of conferences between Allied leaders, diplomats, and military officials gradually shaped the makeup of the alliance, the direction of the war, and ultimately the postwar international order.

The Allies became a formalized group upon the Declaration by United Nations on 1 January 1942, which was signed by 26 countries around the world; these ranged from governments in exile from the Axis occupation to small states far removed from the war. The Declaration officially recognized the Big Three and China as the "Four Powers", acknowledging their central role in prosecuting the war; they were also referred to as the "trusteeship of the powerful", and later as the "Four Policemen" of the United Nations. Many more countries joined through to the final days of the war, including colonies and former Axis states. After the war ended, the Allies, and the Declaration that bound them, would become the basis of the modern United Nations.

Following the First World War, the Treaty of Versailles (1919) established the League of Nations in an attempt to create a system of collective security and prevent war. The League's covenant obliged members to protect the political and territorial integrity of all members against aggression. Four of the major allies of the First World War—the United Kingdom, France, Italy and Japan—became permanent members of the league's council. The league, however, was weakened by the failure of the United States to join and by the cumbersome rules for enforcing sanctions for breaches of its security provisions.

France attempted to further protect itself against possible future German attack with the Franco-Polish alliance (1921) and the Franco-Czechoslovakian alliance (1924). Under the Locarno treaties (1925), France, Britain, Belgium, Germany and Italy also guaranteed the borders between Germany and France and Germany and Belgium as defined in the Treaty of Versailles.

The system of collective security was weakened when Japan withdrew from the League in February 1933, following the League's criticism of Japan's invasion of Manchuria in 1931. A further blow came when Nazi Germany withdrew from the League and the world disarmament conference in October 1933. In May 1935, France signed a mutual defence agreement with the Soviet Union, which Germany saw as directed against it. In October 1935, Italy invaded Abyssinia and the league responded with weak, short-lived sanctions.

Germany remilitarised the Rhineland in March 1936 in contravention of the Versailles and Locarno treaties but Britain, France and the League of Nations imposed no sanctions. Britain, however, announced that it would aid France and Belgium if they were the victims of aggression, and France stated that it would assist Britain and Belgium in the same circumstances. In July 1937, Japan began an undeclared war in China. The league found Japan's actions illegal and invited its members to impose sanctions. In November, Italy joined the German and Japanese Anti-Comintern pact, and in December it left the league.