The Soviet Union, much like the United States, became involved in World War II when it was attacked by a country with which it was not officially at war.  Germany and the Soviet Union had signed a non-aggression pact in 1939.  This pact split Eastern Europe into Germany and Soviet spheres of influence.  It also meant that there would presumably be no war between the two countries.  But then, in June of 1941, Hitler attacked the Soviet Union in what was called Operation Barbarossa.  When Hitler did this, the Soviets became involved in the war.