Answer:
Several countries switched sides in WWII, including Romania, Bulgaria, Finland and Italy.
So although the Soviet Union signed a Non-aggression pact in the beginning, Hitler broke the pact in 1941 by invading the U.R.S.S.
Then there was an alliance between U.S.A. and U.R.S.S around 1941 and 1945 with great degree of cooperation in order to defeat Nazi Germany; thereby The Soviet Union became an ally of the United States and Great Britain during World War II.
Step-by-step explanation: