The relationship between the United States and the Soviet Union became tense for several reasons. At the end of World War II, the United States and the Soviet Union had made several agreements. Within weeks of signing those agreements, it appeared the Soviet Union wasn’t following them. For example, the Declaration of Liberated Europe said that countries would be free to choose the kind of government they wanted. Yet the King of Romania said he was pressured to have a communist government in Romania. Additionally, the new government in Poland had very few pre-war members in the new government. We had an agreement there would be free elections in Poland, and there would be members of the pre-war government in it.
After World War II, the Soviet Union wanted to spread its system of communism. The United States wanted to keep communism from spreading. Thus, we opposed the Soviet Union when they tried to force us of West Berlin. We flew supplies over the Berlin Blockade. We opposed the Soviet-supported North Korea when North Korea invaded South Korea in June 1950. We developed an arsenal of nuclear weapons when the Soviets got the ability to create their own nuclear weapons.
The United States opposed the Soviet Union in many ways at the end of and immediately after World War II. This led to increased tensions between these countries.