At the start of the war on 1 September 1939, the Allies consisted of France, Poland and the United Kingdom, and dependent states, such as British India. Within days they were joined by the independent Dominions of the British Commonwealth: Australia, Canada, New Zealand and South Africa. After the start of the German invasion of North Europe till the Balkan Campaign, the Netherlands, Belgium, Greece, and Yugoslavia joined the Allies. After first having cooperated with Germany in invading Poland whilst remaining neutral in the Allied-Axis conflict, the Soviet Union perforce joined the Allies in June 1941 after being invaded by Germany. The United States provided war material and money all along, and officially joined in December 1941 after the Japanese attack on Pearl Harbor. China had already been in a prolonged war with Japan since the Marco Polo Bridge Incident of 1937, but officially joined the Allies in 1941.
The alliance was formalised by the Declaration by United Nations, from 1 January 1942. However, the name United Nations was rarely used to describe the Allies during the war. The leaders of the "Big Three" – the Soviet Union, the United Kingdom, and the United States – controlled Allied strategy; relations between the United Kingdom and the United States were especially close. China and the Big Three were referred as a "trusteeship of the powerful", then were recognized as the Allied "Big Four" in a Declaration by United Nations and later as the "Four Policemen" of the United Nations.
In 1945, the Allied nations became the basis of the United Nations.
The origins of the Allied powers stem from the Allies of World War I and cooperation of the victorious powers at the Paris Peace Conference, 1919. Germany resented signing Treaty of Versailles. The new Weimar Republic's legitimacy became shaken. However, the 1920s were peaceful.
With the Stock Market Crash of 1929 and the ensuing Great Depression, political unrest in Europe soared including the rise in support of revanchist nationalists in Germany who blamed the severity of the economic crisis on the Treaty of Versailles. By the early 1930s, the Nazi Party led by Adolf Hitler became the dominant revanchist movement in Germany and Hitler and the Nazis gained power in 1933. The Nazi regime demanded the immediate cancellation of the Treaty of Versailles and made claims to German-populated Austria, and German-populated territories of Czechoslovakia. The likelihood of war was high, and the question was whether it could be avoided through strategies such as appeasement.
In Asia, when Japan seized Manchuria in 1931, the League of Nations condemned it for aggression against China. Japan responded by leaving the League of Nations in March 1933. After four quiet years, the Sino-Japanese War erupted in 1937 with Japanese forces invading China. The League of Nations condemned Japan's actions and initiated sanctions on Japan. The United States, in particular, was angered at Japan and sought to support China.
In March 1939, Germany took over Czechoslovakia, violating the Munich Agreement signed six months before, and demonstrating that the appeasement policy was a failure. Britain and France decided that Hitler had no intention to uphold diplomatic agreements and responded by preparing for war. On 31 March 1939, Britain formed the Anglo-Polish military alliance in an effort to avert a German attack on the country. Also, the French had a long-standing alliance with Poland since 1921. The Soviet Union sought an alliance with the western powers, but Hitler ended the risk of a war with Stalin by signing the Nazi–Soviet non-aggression pact in August 1939. The agreement secretly divided the independent nations of Eastern Europe between the two powers and assured adequate oil supplies for the German war machine. On 1 September 1939, Germany invaded Poland; two days later Britain and France declared war on Germany. Then, on 17 September 1939, the Soviet Union invaded Poland from the east. A Polish government-in-exile was set up and it continued to be one of the Allies, a model followed by other occupied countries. After a quiet winter, Germany in April 1940 invaded and quickly defeated Denmark, Norway, Belgium, the Netherlands and France. Britain and its Empire stood alone against Hitler and Mussolini. In June 1941, Hitler broke the non-aggression agreement with Stalin and Germany invaded the Soviet Union. In December, Japan attacked the US and Britain. The main lines of World War II had formed.