How did the united sates and the soviets end up on opposite sides

Because of World War II there was an agreement between the United States, Great Britain, and the Soviet Union called the Yalta agreement. The agreement was brought to bring the Soviet Union to declare war on Japan, this becoming an ally with the United States and Great Britain. The agreement was that the Soviet Union to gain back control of the territories that the Soviet had lost during a different war in 1905 known as the Russo Japanese War.