During World War II, the United States and the Soviet Union were allies, and they worked together in order to defeat the nations of the Axis. However, after the war, and particularly after the Yalta Conference, it became obvious that the two nations had very different ideas of what the future world should look like. The very different approaches that each side took towards its goals made these differences obvious. It also encouraged the idea that the two countries were enemies. This enmity was expressed through the conflicting ideas of capitalism and communism.
As the rise of communism became more feared, distrust and suspicion grew within the United States. People believed that the United States needed to actively protect the world from the threat of communism. This led to the beginning of what is known as the Cold War.