Ku Klux Klan, the name of three movements that became prominent in the United States, one just after the Civil War, another in the 1920's, and a third beginning in the late 1950's.

The first Ku Klux Klan, formed in Pulaski, Tennessee, in 1865, was a secret organization of Southern whites who used intimidation and violence to maintain white supremacy in their states after the slaves were freed. Wearing robes, hoods, and masks that gave them a ghostlike appearance, Klansmen rode through the night, terrorizing ex-slaves and whites who were sympathetic to the cause of the freed slaves. In 1871 Congress authorized the use of federal troops to suppress the Klan, but it existed in some areas until Reconstruction ended in 1877. At its peak, the Klan had 550,000 members.

The second Ku Klux Klan was formed in 1915 at Atlanta, Georgia. The revival was planned to coincide with the opening in Atlanta of D. W. Griffith's motion picture The Birth of a Nation , which glorified the first Klan. The second Klan became influential during a period of intense nationalism after World War I. The Klan's hooded members— native white Protestants—claimed to be patriotic in opposing blacks, Catholics, Jews, and foreign-born Americans. Fiery crosses were set in front of homes as warnings, and acts of violence, including lynchings, were committed. The Klan became politically powerful in a number of states. In 1924 it claimed a membership of 5,000,000 in 40 states. By the 1930's, after being denounced by the Supreme Court, investigated by Congress, and legislated against in various states, it had a membership of only about 30,000. The organization disbanded in 1944.

Attempts to revive a national Ku Klux Klan after World War II failed, but regional Klans were formed in the South. They remained small until the late 1950's, when a Klan resurgence began in reaction to the civil rights movement. Klansmen were implicated in bombings, beatings, and murders in the South after 1960. Some Klan activity also was reported in northern states. Membership grew after passage of civil rights laws in the 1960's. President Johnson denounced the Klan in 1965 and a Congressional investigation followed. Active membership in various Klans was estimated at 17,000 in 1967, but has steadily declined ever since. In the 1990's the Klan's membership was estimated at between 5,000 and 6,000.