Tension after World War ll was high between the United States and the Soviet Union. It then increased whenever The Marshall Plan was introduced by the United States.
They also had different views on Communism and Capitalism. The Soviet Union stood for Communism while the Americans stood for Capitalism.
The Soviet Union wanted to spread Communism while the United States had nuclear weapons. They both feared one another.
The United States created the Truman Doctrine which would support any country that would resist Communism. The Soviets then created the Comecon, which provided aid to their allies who turned down The United States
The Cold War went on for 40 years because the United States and the Soviets never had their military fight. This war was based on ideas.