The Soviet Union joined the Allied Forces because Germany attacked them.
The U.S - Soviet Alliance of 1941-1945 was created because it is essential in securing the defeat of Nazi Germany.
Though
Allied Forces regard the Soviet Union as their enemies, President
Franklin Roosevelt recognizes Germany as a greater threat. The Allies
tried to form an alliance with the Soviet Union but was faced with
demands and conditions that they did not agree to. Soviet Union finally
agreed to join forces with the Allies when Nazi Germans launched an
invasion to the Soviet Union on June 1941
