The Ku Klux Klan is a secret organization that originated in the southern states during the period of Radical Reconstruction following the American Civil War; the clan was soon reactivated on a wider geographic basis in the 20th century. The original clan was organized in Pulaski, Tennessee, during the winter of 1865 to 1866, by former Confederate army officers who gave their clan a name adapted from the Greek word kuklos, which means circle. Although the Ku Klux Klan began as a mischievous social organization, its activities soon were directed against the Reconstruction governments and their leaders, both blacks and whites.

The men regarded the Reconstruction governments as hostile and confining. They also believed in the innate weakness of blacks and therefore mistrusted and resented the rise of former slaves to a status of civil equality and often to positions of political power. So the clan became an illegal organization committed to destroying the Reconstruction governments from North and South Carolina to Arkansas. Dressed in robes and wearing masks topped with pointed hoods, the men terrorized public officials in efforts to drive them from office and blacks to prevent them from voting, holding office. It was customary for the men to burn crosses on hillsides and near the homes of those they wished to frighten. When such tactics failed their victims might be beaten or murdered.

From 1868 to 1870 the clan was increasingly dominated by the rougher elements in the population. The local organizations were called klaverns, these became so uncontrollable and violent that the Grand Wizard, former Confederate general Nathan B. Forrest, officially disbanded the clan in 1869. Klaverns, however, continued to operate on their own. In 1871, Congress passed the Force Bill to implement the 14th Amendment to the U.S. Constitution guaranteeing the rights of all citizens. In the same year President Ulysses S. Grant issued a proclamation calling on members of illegal organizations to disarm and disband; afterwards hundreds of men were arrested. The remaining klaverns gradually faded as the political and social class of blacks was reestablished.

The name, rituals, and some of the attitudes of the original clan were adopted by a new fraternal organization incorporated in Georgia in 1915. The official name of the new society, Knights of the Ku Klux Klan, which was organized by a former preacher, Colonel William Simmons. Membership was open to native-born, white, Protestant males, 16 years of age or older; blacks, Roman Catholics, and Jews were excluded and were increasingly made targets of persecution by the clan. Until 1920 the society exercised little influence. Then, in the period of the Great Depression and political and social unrest that followed World War I, the clan expanded rapidly in urban areas and became active in many states, prominently Colorado, Oregon, Kansas, Oklahoma, Texas, Alabama, Georgia, Illinois, Indiana, Ohio, Pennsylvania, and New Jersey. Although the clan fiercely preached white supremacy, it focused its attack on what it considered to be alien, the Roman Catholics, which it believed was threatening traditional American ways and values. All non-Protestants, aliens, and liberals were denounced as treasonous.

Like its model, the clan burned fiery crosses to frighten its victims. Masked Men also marched through the streets of many communities, carrying posters threatening various people with punishment and warning others to leave town. Many people were kidnapped, beaten, and mutilated by the clan; a number were killed. Few prosecutions of men resulted, and in some communities they were aided by local officials.

Journalistic disclosures of crimes committed by the clan and of corruption and immorality in its leadership led to a congressional investigation in 1921, and for a time the clan changed its tactics. After 1921 it experienced a rapid growth of membership and became politically influential throughout the nation. One estimate of its membership, when the clan was at the peak of its strength the population was as high as 3 million.

In the mid-1920s, inappropriate leadership, internal conflict, and alleged clan immorality and violence badly damaged the clan's reputation, and political opposition increased. By 1929 the membership had been reduced to only several thousand members. During the economic depression of the 1930s the Ku Klux Klan remained active on a small scale, particularly against trade union organizers in the South. It also threatened blacks with punishment if they tried to exercise their right to vote. In 1940 the clan joined with the German-American Bund, an organization financed in part by the government of Nazi Germany, in holding a large rally at Camp Nordland, New Jersey. After the entry of the United States into World War II, the clan curtailed its activities. In 1944 it disbanded formally when it was unable to pay back taxes owed to the federal government. Revival of clan activities after the war led to widespread public sentiment for the suppression of the organization.