After World War II, the two remaining global superpowers, the Soviet Union and the United States, became intensely suspicious of each other. On the American side, politicians and common people were worried that communist spies were in the process of infiltrating society with the ultimate purpose of overthrowing the government of the United States. There was some truth to this. During the Cold War, Soviet spies did in fact carry out espionage in the United States. One famous example was the case of Julius and Ethel Rosenberg, who were convicted of selling atomic secrets and then executed.
International events that exacerbated American fears of communism included the overthrow of the Chinese government by communist forces, the testing of an atomic weapon by the Russians, and the invasion of South Korea by Soviet and Chinese-backed North Korea.
Some politicians took advantage of U.S. fears of communism to exacerbate the hysteria in the pursuit of personal fame and power. Notable among these fear-mongers was Senator Joseph R. McCarthy. The House Un-American Activities Committee had already been investigating the possibility of communists working inside the federal government and the entertainment industry for some time, but when McCarthy took the spotlight, he raised hearsay, intimidation, and outright lies to a new level during the height of the so-called Red Scare. Anyone who opposed McCarthy was declared disloyal. Americans became convinced that communists had infiltrated every facet of society. As a result, many innocent people lost their jobs, notably in the entertainment industry but also in many other fields.
Domestic politics were shaped for years by the fear of communist infiltration. The political climate became overwhelmingly conservative, as anyone who wanted to achieve success had to demonstrate that they were fervently anticommunist. Free speech and other civil liberties were negatively impacted because people were unable to say what they really thought for fear of being branded communists.
Abroad, the Korean War was fought to contain communism, and later, the United States entered into the conflict in Vietnam in an effort to prevent communism from overrunning Southeast Asia.