Germany caused the United States and Soviet Union to be allies for a period of time between 1941 and 1945.
Although the relationship between the United States and the Soviet Union had been strained for many years before World War II, they became allies to defeat Nazi Germany. The main reason the Soviets entered into an alliance with the United States is because of Nazi descision to launch its invasion of the Soviet Union in June, 1941. Without the help of the Soviet Union, the United States and Great Britain would have had difficulties achieving a military victory over Germany.
