Because of World War II there was an agreement between the United States, Great Britain, and the Soviet Union called the Yalta agreement. This agreement was not to be as it was originally viewed. The agreement was brought to view to bring the Soviet Union to declare war on Japan, thus becoming an ally with the United States and Great Britain. The agreement promised the Soviet Union to gain back control of the European territories that the Soviet had lost during a different war in 1905 known as the Russo-Japanese War.
The idea, in the mind of then President of the United States, Franklin D. Roosevelt, was that these territories would be liberated by being given back to the Soviet Union. The problem with this agreement is when the Soviet Union took back these territories, as promised, they actually moved into the territories. The Soviet Union was afraid of another country attempting to take over, such as Germany had attempted to do in World War II. The Soviet Union decided to do what it had to do to prevent this from ever happening.
The idea of liberation for these territories became a delusion because when the Soviet Union began to occupy these territories, the United States and Great Britain found it very difficult to negotiate any more with the Soviet. The Soviet Union, instead of liberating these territories, brought into them communism.
The United States felt that they had been betrayed by the Soviet Union when the Soviet decided to be self-preserving instead of acting like the ally they were supposed to be according to the idea within the Yalta agreement.
Beyond the Yalta agreement turmoil, the United States found evidence that they Soviet Union had been spying on them during the time they were supposed to have been allies. In the end, both side lost trust in each other and became enemies.