The four major Allies of WWII were Great Britain, France, the United States and Russia. France was an Allied Power with the exception of the time period between 1940 to 1944 when it was under German occupation. France and Great Britain were the first to declare war against Germany after German forces started occupying parts of eastern Europe, such as Poland and Czechoslovakia in 1939.
Russia entered the war in June of 1941. The United States, which was an isolationist country at that time, became an Allied Power after the bombing of Pearl Harbor by the Japanese in December of 1941. Although these were the main allies in WWII, there were many other nations that were also allies, such as China, Australia, Canada and New Zealand.