How did the Soviet Union become allied with the US during the World War II?

The Soviet Union became an Allied country when it was invaded by Germany on June 22, 1941. Germany and Italy declared war on the US on December 11, 1941; three days after the US declaration of war against their ally Japan. However, the Soviet Union did not move against Japan until near the end of the war, and only then because the US required it in return for Soviet territorial demands in Eastern Europe.