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

Why this is the answer

Two answers are accepted: communism and nuclear war. Communism, as the USCIS study guide explains it, is a system in which the government plans the economy and controls most resources, and in most communist countries a single party controls the government. The Soviet Union was communist, and during the Cold War China and North Korea were heading the same way. The United States wanted to stop other countries around the world from becoming communist. American leaders worried that if one country fell, its neighbors would follow. In 1947 Undersecretary of State Dean Acheson made that case to members of Congress, warning that if Greece and Turkey fell, communism could spread to Iran and as far as India. The idea became known as the domino theory. Together with the Truman Doctrine, it guided American policy for decades, and later administrations used similar reasoning to justify action in Korea, Cuba and Vietnam. The second fear was nuclear war. The United States had dropped two atomic bombs on Japan in 1945. During the Cold War the United States and the Soviet Union built large armies in case they went to war, and the testing of atomic and then hydrogen weapons continued. In October 1962 American U-2 spy planes photographed Soviet medium-range nuclear missiles in Cuba, able to strike major American cities within minutes. On October 28 the Soviets agreed to remove them, and the immediate threat of nuclear war passed. On August 5, 1963, the United States, Britain and the Soviet Union signed the Limited Test Ban Treaty, which banned nuclear tests in the atmosphere, in outer space and under water. Either answer is enough. The study guide names the spread of communism as the main concern, but USCIS accepts nuclear war just the same. A mistake to avoid is answering "the Soviet Union" or "Russia." That is the answer to the question about the main rival. The rival was a country; the concerns were a political system and a kind of war.