After World War II, the US and Soviet Union emerged as global superpowers. However, they became bitter rivals despite their alliance during World War II. This was mainly due to their differences on how a society should be run. The US was a democratic/capitalist country while the Soviet Union was communist.
After World War II, the US made it a policy to try to limit the power of the Soviet Union and communism as much as possible. This lead to the foreign policy of containment, which focused on stopping the spread of communism. Understanding that America's foreign policy is based around containment, it is easy to understand why they disliked the Soviet Union taking over these countries. The US did not want the Soviet Union's power and influence to spread, as America was afraid that it would continue to spread to other countries (also known as the Domino effect).
