The most significant Allied countries in World War II were Great Britain and the Commonwealth, France, the Soviet Union, the United States and China, though, in formal terms, the Allies included all wartime members of the United Nations and all the original signatories of their declaration. The Allied coalition was united militarily against the primary Axis forces of Germany, Italy and Japan.
The first incarnation of the coalition consisted of Poland, France and then Great Britain and called for France and Britain to intervene should Poland be attacked by Germany. This situation occurred at the beginning of September 1939, with Poland being swiftly conquered. Soon afterward, Germany subordinated much of France, leaving Britain on its own against Axis forces in the West as well as in its Asian outposts. By 1940, however, the German invasion of Russia nullified its former pact with the Soviets, leading to Russia's introduction into the Allied coalition.
Although the United States offered some logistical and financial support beforehand, it wasn't formally dragged into the war on the Allied side until December 1941 with the Japanese raid on Pearl Harbor. That action subsequently forced Japan's chief allies, Germany and Italy, to declare war on the United States. China saw the longest war of all the Allied states, starting with the 1937 invasion by the Japanese in Manchuria. Throughout that time, China received strong material and political support from its allies, particularly the United States and the British Commonwealth.