Claim: World War II was fought between the United States and Japan.

The Allies of World War II , called the United Nations from the 1 January 1942 declaration , were the countries that together opposed the Axis powers during the Second World War -LRB- 1939 -- 1945 -RRB- . The Allies promoted the alliance as seeking to stop German , Japanese and Italian aggression .   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 the British India . Within days they were joined by the independent Dominions of the British Commonwealth : Australia , Canada , New Zealand and South Africa . Poland was a minor factor after its defeat in 1939 ; France was a minor factor after its defeat in 1940 . From the German invasion of North Europe to the Balkan Campaign , 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 materiel and money all along , and officially joined in December 1941 after the Japanese attack on Pearl Harbor . China had already been into a prolonged war with Japan since the Lugou 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 UK , the Soviet Union , 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 Declaration by United Nations and later the `` Four Policemen '' of the United Nations .   In 1945 , the Allied nations became the basis of the United Nations .