The first Klan flourished in the Southern United States in the late 1860s, then died out by the early 1870s. It sought to overthrow the Republican state governments in the South during the Reconstruction Era, especially by using violence against African-American Leaders. With numerous autonomous chapters across the South, it was suppressed around 1871, through federal law enforcement. Members made their own, often colorful, costumes: robes, masks and conical hats, designed to be terrifying and to hide their identities.

Six Confederate veterans from Pulaski, Tennessee created the original Ku Klux Klan on December 24, 1865, during the Reconstruction of the South after the Civil War. The name was formed by combining the Greek kyklos (κύκλος, circle) with clan. The group was known for a short time as the "Kuklux Clan". The Ku Klux Klan was one of a number of secret, oath-bound organizations using violence, which included the Southern Cross in New Orleans (1865) and the Knights of the White Camelia (1867) in Louisiana.

Historians generally classify the KKK as part of the post-Civil War insurgent violence related not only to the high number of veterans in the population, but also to their effort to control the dramatically changed social situation by using extrajudicial means to restore white supremacy. In 1866, Mississippi Governor william L. Sharkey reported that disorder, lack of control, and lawlessness were widespread; in some states armed bands of Confederate Soldiers roamed at will. The Klan used public violence against black people and their allies as intimidation. They burned houses and attacked and killed black people, leaving their bodies on the roads.

"Armed guerrilla warfare killed thousands of Negroes; political riots were staged; their causes or occasions were always obscure, their results always certain: ten to one hundred times as many Negroes were killed as whites." Masked men shot into houses and burned them, sometimes with the occupants still inside. They drove successful black farmers off their land. "Generally, it can be reported that in North and South Carolina, in 18 months ending in June 1867, there were 197 murders and 548 cases of aggravated assault."

By 1868, two years after the Klan's creation, its activity was beginning to decrease. Members were hiding behind Klan masks and robes as a way to avoid prosecution for freelance violence. Many influential southern Democrats feared that Klan lawlessness provided an excuse for the federal government to retain its power over the South, and they began to turn against it. There were outlandish claims made, such as Georgian B. H. Hill stating "that some of these outrages were actually perpetrated by the political friends of the parties slain."

A KKK group was even established in Fiji in the early 1870s by white settlers, although it was put down by the British who, although not officially established as Fiji's colonial rulers, had played a leading role in establishing a new constitutional monarchy that was being threatened by the Fijian Klan.

In 1871, President Ulysses S. Grant signed Butler's legislation. The Ku Klux Klan Act and the Enforcement Act of 1870 were used by the federal government to enforce the civil rights provisions for individuals under the constitution. The Klan refused to voluntarily dissolve after the 1871 Klan Act, so President Grant issued a suspension of habeas corpus and stationed Federal troops in nine South Carolina counties. The Klansmen were apprehended and prosecuted in federal court. Judges Hugh Lennox Bond and George S. Bryan presided over the trial of KKK members in Columbia, South Carolina during December 1871. The defendants were given from three months to five years of incarceration with fines. More blacks served on juries in Federal court than on local or state juries, so they had a chance to participate in the process. Hundreds of Klan members were fined or imprisoned during the crackdown.

After the Klan was suppressed, similar insurgent paramilitary groups arose that were explicitly directed at suppressing Republican voting and turning Republicans out of office: the White League, which started in Louisiana in 1874; and the Red Shirts, which started in Mississippi and developed chapters in the Carolinas. For instance, the Red Shirts are credited with helping elect Wade Hampton as governor in South Carolina. They were described as acting as the military arm of the Democratic Party and are attributed with helping white Democrats regain control of state legislatures throughout the South. In addition, there were thousands of Confederate veterans in what were called rifle clubs.

In 1882, the Supreme Court ruled in United States v. Harris that the Klan Act was partially unconstitutional. It ruled that Congress's power under the Fourteenth Amendment did not include the right to regulate against private conspiracies. It recommended that persons who had been victimized should seek relief in state courts, which were entirely unsympathetic to such appeals.

In the 1890s the Filipino Katipunan society, which shared the KKK acronym, was inspired by the experience of the first Klan, although it did not have any affiliation with the Klan, and it never espoused racism or committed hate crimes against non-Filipinos.

According to The Cyclopædia of Fraternities (1907), "Beginning in April, 1867, there was a gradual transformation ... The members had conjured up a veritable Frankenstein. They had played with an engine of power and mystery, though organized on entirely innocent lines, and found themselves overcome by a belief that something must lie behind it all — that there was, after all, a serious purpose, a work for the Klan to do."

All of the above terminology was created by william Joseph Simmons, as part of his 1915 revival of the Klan. The Reconstruction-era Klan used different titles; the only titles to carry over were "Wizard" for the overall leader of the Klan and "Night Hawk" for the official in charge of security.

The Second Klan saw threats from every direction. According to Historian Brian R. Farmer, "two-thirds of the national Klan lecturers were Protestant ministers". Much of the Klan's Energy went into guarding the home, and Historian Kathleen Blee says that its members wanted to protect "the interests of white womanhood". Joseph Simmons published the pamphlet ABC of the Invisible Empire in Atlanta in 1917; in it, he identified the Klan's goals as "to shield the sanctity of the home and the chastity of womanhood; to maintain white supremacy; to teach and faithfully inculcate a high spiritual philosophy through an exalted ritualism; and by a practical devotedness to conserve, protect and maintain the distinctive institutions, rights, privileges, principles and ideals of a pure Americanism". Such moral-sounding purpose underlay its appeal as a fraternal organization, recruiting members with a promise of aid in settling in the new urban societies of rapidly growing cities such as Dallas and Detroit.

Studies developed as social history find that in general, the KKK membership in these cities was from the stable, successful middle classes, with few members drawn from the elite or the working classes. Pegram, reviewing the studies, concludes, "the popular Klan of the 1920s, while diverse, was more of a civic exponent of white Protestant social values than a repressive hate group."

Northern Indiana's industrial cities had attracted a large Catholic population of European immigrants and their descendants. They established the University of Notre Dame, a major Catholic college near South Bend. In May 1924 when the KKK scheduled a regional meeting in the city, Notre Dame students blocked the Klansmen and stole some KKK regalia. The next day the Klansmen counterattacked. Finally the college President and the football coach Knute Rockne kept the students on campus to avert further violence.

In Indiana, traditional political historians focused on notorious Leaders, especially D. C. Stephenson, the Grand Dragon of the Indiana Klan, whose conviction for 1925 kidnap, rape, and murder of Madge Oberholtzer helped destroy the Ku Klux Klan movement nationwide. In his history of 1967, Kenneth Jackson already described the Klan of the 1920s as associated with cities and urbanization, with chapters often acting as a kind of fraternal organization to aid people coming from other areas.

Scholars and biographers have recently examined Hugo Black's Klan role. Ball finds regarding the KKK that Black "sympathized with the group's economic, nativist, and anti-Catholic beliefs." Newman says Black "disliked the Catholic Church as an institution" and gave over 100 anti-Catholic speeches in his 1926 election campaign to KKK meetings across Alabama. Black was elected US senator in 1926 as a Democrat. In 1937 President Franklin D. Roosevelt appointed Black to the Supreme Court without knowing how active in the Klan he had been in the 1920s. He was confirmed by his fellow Senators before the full KKK connection was known; Justice Black said he left the Klan when he became a senator.

In Alabama, KKK vigilantes launched a wave of physical terror in 1927. They targeted both blacks and whites for violations of racial norms and for perceived moral lapses. This led to a strong backlash, beginning in the media. Grover C. Hall, Sr., Editor of the Montgomery Advertiser from 1926, wrote a series of editorials and articles that attacked the Klan. (Today the paper says it "waged war on the resurgent [KKK]".) Hall won a Pulitzer Prize for the crusade, the 1928 Editorial Writing Pulitzer, citing "his editorials against gangsterism, floggings and racial and religious intolerance." Other newspapers kept up a steady, loud attack on the Klan, referring to the organization as violent and "un-American". Sheriffs cracked down on activities. In the 1928 presidential election, the state voters overcame their initial opposition to the Catholic candidate Al Smith, and voted the Democratic Party line as usual.

Although in decline, a measure of the Klan's influence was still evident when it staged its march along Pennsylvania Avenue in Washington, D.C. in 1928. By 1930 Klan membership in Alabama dropped to less than 6,000. Small independent units continued to be active in the industrial city of Birmingham.

In major southern cities such as Birmingham, Alabama, Klan members kept control of access to the better-paying industrial jobs and opposed unions. During the 1930s and 1940s, Klan Leaders urged members to disrupt the Congress of Industrial Organizations (CIO), which advocated industrial unions and accepted African-American members, unlike earlier unions. With access to dynamite and using the skills from their jobs in mining and steel, in the late 1940s some Klan members in Birmingham used bombings to destroy houses in order to intimidate upwardly mobile blacks who moved into middle-class neighborhoods. "By mid-1949, there were so many charred house carcasses that the area [College Hills] was informally named Dynamite Hill."

In 1939, after experiencing several years of decline due to the Great Depression, the Imperial Wizard Hiram Wesley Evans sold the national organization to James A. Colescott, an Indiana veterinary physician, and Samuel Green, an Atlanta obstetrician. They could not revive the Klan's declining membership. In 1944, the Internal Revenue Service filed a lien for $685,000 in back taxes against the Klan, and Colescott dissolved the organization that year. Local Klan groups closed down over the following years.

Racial terrorism was used in smaller towns to suppress black political activity; Elbert Williams of Brownsville, Tennessee was lynched in 1940 for trying to organize black residents to register and vote. That year, Jesse Thornton of Luverne, Alabama was lynched for failing to address a police officer as "Mister."

Among the more notorious murders by Klan members in the 1950s and 1960s:

According to a report from the Southern Regional Council in Atlanta, the homes of 40 black Southern families were bombed during 1951 and 1952. Some of the bombing victims were social Activists whose work exposed them to danger, but most were either people who refused to bow to racist convention or were innocent bystanders, unsuspecting victims of random violence.

There was considerable resistance among African Americans and white allies to the Klan. In 1953, newspaper publishers W. Horace Carter (Tabor City, North Carolina), who had campaigned for three years, and Willard Cole (Whiteville, North Carolina) shared the Pulitzer Prize for Public Service citing "their successful campaign against the Ku Klux Klan, waged on their own doorstep at the risk of economic loss and personal danger, culminating in the conviction of over one hundred Klansmen and an end to terrorism in their communities." In a 1958 incident in North Carolina, the Klan burned crosses at the homes of two Lumbee Native Americans for associating with white people, and threatened more actions. When the KKK held a nighttime rally nearby, they were quickly surrounded by hundreds of armed Lumbee. Gunfire was exchanged, and the Klan was routed at what became known as the Battle of Hayes Pond.

Recruitment activity has also been reported in the United Kingdom, dating back to the 1960s when Robert Relf was involved in establishing a British KKK.

During the tenure of Bull Connor as police commissioner in Birmingham, Klan groups were closely allied with the police and operated with impunity. When the Freedom Riders arrived in Birmingham in 1961, Connor gave Klan members fifteen minutes to attack the riders before sending in the police to quell the attack. When local and state authorities failed to protect the Freedom Riders and Activists, the federal government began to establish intervention and protection.

As 20th-century Supreme Court rulings extended federal enforcement of citizens' civil rights, the government revived the Enforcement Acts and the Klan Act from Reconstruction days. Federal prosecutors used these laws as the basis for investigations and indictments in the 1964 murders of Chaney, Goodman, and Schwerner; and the 1965 murder of Viola Liuzzo. They were also the basis for prosecution in 1991 in Bray v. Alexandria Women's Health Clinic.

In 1965, the House Un-American Activities Committee started an investigation on the Klan, putting in the public spotlight its front organizations, finances, methods and divisions.

Jerry Thompson, a newspaper reporter who infiltrated the KKK in 1979, reported that the FBI's COINTELPRO efforts were highly successful. Rival KKK factions accused each other's Leaders of being FBI informants. william Wilkinson of the Invisible Empire, Knights of the Ku Klux Klan, was revealed to have been working for the FBI.

Duke has an account on Stormfront which he uses to post articles from his own website. He also polls forum members for opinions and questions, in particular during his internet broadcasts. Duke has worked with Don Black on numerous projects including Operation Red Dog in 1980.

In Australia in the late 1990s, former One Nation member Peter Coleman established branches throughout the country, and circa 2012 the KKK has attempted to infiltrate other political parties such as Australia First.

In 1995, Don Black and Chloê Hardin, the ex-wife of the KKK Grand Wizard David Duke, began a small bulletin board system (BBS) called Stormfront. In the 21st century, Stormfront has become a prominent online forum for white nationalism, Neo-Nazism, hate speech, racism, and antisemitism.

After Michael Donald was lynched in 1981 in Alabama, the FBI investigated his death. The US Attorney prosecuted the case. Two local KKK members were convicted for his murder, including Henry Francis Hays, who was sentenced to death. With the support of attorneys Morris Dees of the Southern Poverty Law Center (SPLC) and State Senator Michael A. Figures, Donald's mother Beulah Mae Donald sued the KKK in civil court in Alabama. Her lawsuit against the United Klans of America was tried in February 1987. After exhausting the appeals process, Hays was executed by electric chair for Donald's death in Alabama on June 6, 1997. It was the first time since 1913 that a white man had been executed in Alabama for a crime against an African American.

The modern KKK is not one organization; rather it is composed of small independent chapters across the United States. According to a 1999 ADL report, the KKK's estimated size then was "No more than a few thousand, organized into slightly more than 100 units." In 2017, the Southern Poverty Law Center (SPLC), which monitors extremist groups, estimated that there were "at least 29 separate, rival Klan groups currently active in the United States, and they compete with one another for members, dues, news media attention and the title of being the true heir to the Ku Klux Klan." The formation of independent chapters has made KKK groups more difficult to infiltrate, and researchers find it hard to estimate their numbers. Analysts believe that about two-thirds of KKK members are concentrated in the Southern United States, with another third situated primarily in the lower Midwest.

In São Paulo, Brazil, the website of a group called Imperial Klans of Brazil was shut down in 2003, and the group's leader was arrested.

Recent KKK membership campaigns have stimulated people's anxieties about illegal immigration, urban crime, civil unions, and same-sex marriage. In 2006 J. Keith Akins argued that, "Klan literature and propaganda is rabidly homophobic and encourages violence against gays and lesbians ... Since the late 1970s, the Klan has increasingly focused its ire on this previously ignored population."

In Germany a KKK-related group, the European White Knights of the Ku Klux Klan, has organised and it gained notoriety in 2012 when it was widely reported in the German media that two police officers who held membership in the organisation would be allowed to keep their jobs.

Catholic and liberal Democrats—who were strongest in northeastern cities—decided to make the Klan an issue at the 1924 Democratic National Convention in New York City. Their delegates proposed a resolution indirectly attacking the Klan; it was defeated by one vote out of 1100. The leading presidential candidates were william Gibbs McAdoo, a Protestant with a base in the South and West where the Klan was strong, and New York Governor Al Smith, a Catholic with a base in the large cities. After weeks of stalemate and bitter argumentation, both candidates withdrew in favor of a compromise candidate.

In 2015, however, the number of KKK chapters nationwide grew from 72 to 190. The SPLC released a similar report stating that "there were significant increases in Klan as well as black separatist groups."

According to a 2016 analysis by the SPLC, hate groups in general are on the rise in the United States. The ADL published a report in 2016 that concluded: "Despite a persistent ability to attract media attention, organized Ku Klux Klan groups are actually continuing a long-term trend of decline. They remain a collection of mostly small, disjointed groups that continually change in name and leadership."

The Imperial Wizard of the Traditionalist American Knights, Frank Ancona, was fatally shot in Missouri in February 2017, several days after disappearing. The coroner declared his death a homicide. Ancona's wife and stepson were charged with first-degree murder in connection with the killing. The prosecutor in the case believes that the killing "happened because of a marital dispute" and was not connected to Ancona's Klan participation. Ancona's group "was not considered the largest or the most influential iteration of the Klan, but he was skilled at attracting the spotlight."

The second and third incarnations of the Ku Klux Klan made frequent references to America's "Anglo-Saxon" blood, hearkening back to 19th-century nativism. Although members of the KKK swear to uphold Christian morality, virtually every Christian denomination has officially denounced the KKK.