Why did the united states engage in the cold war with the authoritarian soviet union?

Although relations between the Soviet Union and the United States had been strained in the years before World War II, the U.S.-Soviet alliance of 1941–1945 was marked by a great degree of cooperation and was essential to securing the defeat of Nazi Germany. Without the remarkable efforts of the Soviet Union on the Eastern Front, the United States and Great Britain would have been hard pressed to score a decisive military victory over Nazi Germany.