During the Cold War, what was the main concern of the United States?

Why this is the answer

One word, communism, is the official answer, and the idea behind it is the fear that communism would spread. The roots go back to World War II. The United States and the Soviet Union, the name for Russia together with the nearby countries it controlled, had fought on the same side, yet they did not trust each other, and once the war ended that distrust hardened into the Cold War. It would last more than forty years. In the words of the USCIS study materials, communism is a system in which the government plans the economy and controls most resources, and in most communist countries a single political party runs the government. The United States and its allies held a different view: that democratic government and a capitalist economy were the best protection for individual rights and freedoms. The Soviet Union was communist, China and North Korea were moving the same way, and American policy aimed to keep other countries from following. In this war, American and Soviet soldiers never fought each other in battle. The rivalry ran through diplomats, spies and threats, and through a race into space, in which the Soviets launched the first satellite in 1957 and the United States landed the first person on the moon in 1969. Where shooting did break out, the same concern lay underneath it. In Korea in the early 1950s, and in Vietnam from 1959 to 1975, American forces backed governments in the south against communist pressure from the north. The end came in three steps: the Berlin Wall fell in 1989, East and West Germany reunited in 1990, and the Soviet Union broke apart in 1991. It is tempting to answer “the Soviet Union” or “Russia.” That names the rival, not the concern, and the question asks what worried the United States. Terrorism is another trap, since it belongs to the September 11 question further down the list. Answer with the idea rather than the country, and you will match the official wording.