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
