When were the US and Soviet Union allies?
The United States and the Soviet Union were allies during World War II, specifically between the years 1941-1945.
That alliance was between the United States, the Soviet Union, and Great Britain. Together they were known as "The Big Three" or simply "The Allies." To clarify, the Big Three were the Allies and Germany was the Axis.
The alliance between the United States and Britain with the Soviet Union was absolutely critical to their ability to defeat Nazi Germany. If the Soviet Union had not hampered Hitler's army on the Eastern front, Germany would have been able to focus its attention on the Western front. The fact that Germany was fighting a two-front war sapped a tremendous amount of resources from what they could have been able to place on a single front.
As soon as World War II ended, though, that alliance was over and in came the Cold War.