The Cold War was not a physical war, but it was 50 years of the United States and the Soviet Union being at the brink of war. They were constantly competing to be the first and best in all things. Their militaries were ready to strike at all times, which took a toll on both nations.
The Soviet Union was a communist nation and the US was thriving with democracy. This caused a lot of conflict between the two with their differing political ideas.
The Soviet Union refused to accept US aid. They did this because they didn't want the US to have control over their economy.
The US offered aid to the countries who were close to falling to communism to lesson Stalin's influence. This became known as the Truman Doctrine.
Tensions rose when the US joined NATO. As a result of this, a few months later the Soviets joined the Warsaw Pact.