The first Red Scare occurred in the wake of World War I. The Russian Revolution of 1917 saw the Bolsheviks, led by Vladimir Lenin, topple the Romanov dynasty, kicking off the rise of the communist party and inspiring international fear of Bolsheviks and anarchists.

In the United States, labor strikes were on the rise, and the press sensationalized them as being caused by immigrants bent on bringing down the American way of life. The Sedition Act of 1918 targeted people who criticized the government, monitoring radicals and labor union leaders with the threat of deportation.

The fear turned to violence with the 1919 anarchist bombings, a series of bombs targeting law enforcement and government officials. Bombs went off in a wide number of cities including Boston, Cleveland, Philadelphia, D.C., and New York City.

The first Red Scare climaxed in 1919 and 1920, when United States Attorney General Alexander Mitchell Palmer ordered the Palmer raids, a series of violent law-enforcement raids targeting leftist radicals and anarchists. They kicked off a period of unrest that became known as the “Red Summer.”

Following World War II (1939-45), the democratic United States and the communist Soviet Union became engaged in a series of largely political and economic clashes known as the Cold War. The intense rivalry between the two superpowers raised concerns in the United States that Communists and leftist sympathizers inside America might actively work as Soviet spies and pose a threat to U.S. security.

Such ideas were not totally unfounded. The Union of Soviet Socialist Republics (USSR) had long carried out espionage activities inside America with the aid of U.S. citizens, particularly during World War II. As apprehension about Soviet influence grew as the Cold War heated up, U.S. leaders decided to take action. On March 21, 1947, President Harry S. Truman (1884-1972) issued Executive Order 9835, also known as the Loyalty Order, which mandated that all federal employees be analyzed to determine whether they were sufficiently loyal to the government. Truman’s loyalty program was a startling development for a country that prized the concepts of personal liberty and freedom of political organization. Yet it was only one of many questionable activities that occurred during the period of anticommunist hysteria known as the Red Scare.

One of the pioneering efforts to investigate communist activities took place in the U.S. House of Representatives, where the House Un-American Activities Committee (HUAC) was formed in 1938. HUAC’s investigations frequently focused on exposing Communists working inside the federal government or subversive elements working in the Hollywood film industry, and the committee gained new momentum following World War II, as the Cold War began. Under pressure from the negative publicity aimed at their studios, movie executives created Hollywood blacklists that barred suspected radicals from employment; similar lists were also established in other industries.

Another congressional investigator, U.S. Senator Joseph R. McCarthy (1908-57) of Wisconsin, became the person most closely associated with the anticommunist crusade–and with its excesses. McCarthy used hearsay and intimidation to establish himself as a powerful and feared figure in American politics. He leveled charges of disloyalty at celebrities, intellectuals and anyone who disagreed with his political views, costing many of his victims their reputations and jobs. McCarthy’s reign of terror continued until his colleagues formally denounced his tactics in 1954 during the Army-McCarthy hearings, when army lawyer Joseph Welch famously asked McCarthy, “Have you no decency?”

The Federal Bureau of Investigation, or FBI, and its longtime director, J. Edgar Hoover (1895-1972), aided many of the legislative investigations of communist activities. An ardent anticommunist, Hoover had been a key player in an earlier, though less pervasive, Red Scare in the years following World War I (1914-18). With the dawning of the new anticommunist crusade in the late 1940s, Hoover’s agency compiled extensive files on suspected subversives through the use of wiretaps, surveillance and the infiltration of leftist groups.

The information obtained by the FBI proved essential in high-profile legal cases, including the 1949 conviction of 12 prominent leaders of the American Communist Party on charges that they had advocated the overthrow of the government. Moreover, Hoover’s agents helped build the case against Julius Rosenberg (1918-53) and his wife, Ethel Rosenberg (1915-53), who were convicted of espionage in 1951. The Rosenbergs were executed two years later.

Public concerns about communism were heightened by international events. In 1949, the Soviet Union successfully tested a nuclear bomb and communist forces led by Mao Zedong (1893-1976) took control of China. The following year saw the start of the Korean War (1950-53), which engaged U.S. troops in combat against the communist-supported forces of North Korea. The advances of communism around the world convinced many U.S. citizens that there was a real danger of “Reds” taking over their own country. Figures such as McCarthy and Hoover fanned the flames of fear by wildly exaggerating that possibility.

As the Red Scare intensified, its political climate turned increasingly conservative. Elected officials from both major parties sought to portray themselves as staunch anticommunists, and few people dared to criticize the questionable tactics used to persecute suspected radicals. Membership in leftist groups dropped as it became clear that such associations could lead to serious consequences, and dissenting voices from the left side of the political spectrum fell silent on a range of important issues. In judicial affairs, for example, support for free speech and other civil liberties eroded significantly. This trend was symbolized by the 1951 U.S. Supreme Court ruling in Dennis v. United States, which said that the free-speech rights of accused Communists could be restricted because their actions presented a clear and present danger to the government.

Americans also felt the effects of the Red Scare on a personal level, and thousands of alleged communist sympathizers saw their lives disrupted. They were hounded by law enforcement, alienated from friends and family and fired from their jobs. While a small number of the accused may have been aspiring revolutionaries, most others were the victims of false allegations or had done nothing more than exercise their democratic right to join a political party.

Though the climate of fear and repression began to ease in the late 1950s, the Red Scare has continued to influence political debate in the decades since. It is often cited as an example of how unfounded fears can compromise civil liberties.