The United States developed the policy of containment in light of the Soviet Union's expansion. The Soviet Union had been devastated through war. During the Second World War, over twenty million Soviets were killed. Stalin's main goal was to protect the USSR from any future attacks. They wanted to create a buffer zone. This zone would be made of eastern European states that would be under Soviet influence. Their fixation with safety measures was seen as an attempt to spread communism to the United States.
The policy of containment was formed in light of "The Long Telegram". This famous document given to the US government by George F. Kennan. Kennan analyzed the state of the Soviet Union and suggested to the US government that "containing" the Soviet Union was the only way. He made note of the role communism played in the country and saw the USSR as "paranoid", "suspicious" and "aggressive", that the Soviets were expansionists.
The policy of containment was to prevent the spread of communism and to control the situation before it got out of hand. Containment was a strategy used to limit and avert further Soviet expansion and prevent communism from attacking weak countries.
Kennan had said that the Soviet Union could not continue, they had no mechanics for real change and that the regime would eventually have to fall to pieces. He noted that the USSR could go from the strongest country to the most pitiful overnight. Kennan's writing was at the heart of the Truman Doctrine. And at the heart of the Truman Doctrine was the idea of containment. The US believed that if one country were to fall to communism then slowly the region would go down with it. This is also known as the domino theory.
But the tension built...