The role of the United States in the Cold War was the role of the “leader of the free world.” The Cold War was a conflict between communism on the one hand and democracy and capitalism on the other. The United States was the undisputed leader of the democratic and capitalist side of this conflict.
After World War II, the US was by far the richest and most powerful of the countries of the West. Therefore, it naturally assumed a leading role in the Cold War. The US was the country that could afford to spend huge sums of money on its military. It was the country that could afford to spend more to help support other countries both militarily and economically if those countries would side with the US in the Cold War. In these ways, the US played a leading role in the Cold War.