The Ku Klux Klan (KKK) is a white supremacist hate and social terrorism group that targets non-white, non-Protestant, Americans and their allies. After the United States defeated the Confederate States and legally emancipated enslaved people, six Confederate veterans founded the first iteration of the KKK on December 24, 1865 in Pulaski, Tennessee. They founded the Klan in an effort to restore Black disenfranchisement and to undermine the federally implemented Republican government under Reconstruction. The Klan used terrorism to intimidate Black southerners, particularly political leaders, their families, and white supporters. Activity declined after Congress passed the Force Act in 1870 and the KKK Act in 1871.

William Joseph Simmons (1880-1845) revised the Klan at a ceremony on Stone Mountain in DeKalb County, Georgia, in 1915. The movement gained momentum largely because of David Walk “D. W.” Griffith's (1875-1948) popular movie The Birth of a Nation, which was about the first iteration of the Klan. This version of the Klan was structured like a fraternal organization and included aspects of a business system. At its height in the 1920s, there were three to eight million members. In addition to terrorizing Black Americans, the group also targeted Catholics, Jews, and immigrants. Anti-hate groups fought against the KKK, and several factors, including the murder and rape conviction of Klan leader David Curtis "D. C." Stephenson (1891-1966), led to its decline and James A. Colescott (1897–1950) officially disbanding the KKK in 1944.

Membership surged again during the Civil Rights Movement in the 1950s and 1960s. Members targeted Civil Rights activists, often bombing their homes, and regularly avoided conviction. Still, members of the Klan were notably convicted in 1967 of the 1964 murder of Civil Rights workers in Mississippi, and in 1977 and 2000 of murdering Black children in the 1963 bombing of the 16th Street Baptist Church. Membership dropped later in the 20th century, but the Ku Klux Klan still exists in the 21st century.