Following the Civil War, a group of former Confederate soldiers, not being happy with the end result of being forced to accept their former slaves as people just like them, formed an organization that became known as the Ku Klux Klan. The Klan was in essence a terrorist organization, targeting black voters and politicians, as well as anybody who supported them, via violence and murder. The Klan remained active throughout the South until the 1870's, when political fenangling over who won a tight presidential race resulted in the southern states regaining the right to treat their black citizens like shit. By the end of the decade the southern states had passed numerous laws that effectively disenfranchised and segregated African-Americans. The Klan, not having much of a purpose after that, just kind of disappeared.
Fast forward to 1915, when famed director D.W. Griffith released the first full length motion picture, a three hour movie called The Birth of a Nation. Though considered one of the great pioneering films of all time, it was also racist as shit, glorifying the founding of the Klan. One of the people who saw the movie was a man by the name of William Simmons, who thought the whole thing was cool as shit. He soon after founded his own Ku Klux Klan, which just like in the film, involved dressing up in bedsheets, burning crosses, and giving each other crazy titles like Grand Imperial Wizard Dragon. It should probably be mentioned that the original Klan had none of these things. Not being happy with just hating black people, Simmons expanded the list of folks and things the group was against to include Catholics, Jews, Italians, Slavs, drinking alcohol, abortions, labor unions, and slutty women. Pretty much if you weren't a white Anglo-Saxon Protestant you could just go right ahead and fuck off. Like its predecessor, the new Klan spent most of its time beating up, and in some cases even lynching, anybody they didn't like.
Strangely enough, even in the 1920's most Americans, even the racist ones, weren't really down with the idea of just straight up kicking in the teeth of those they didn't like. Sure, they might think that some groups certainly had an ass beating coming, but that didn't mean they wanted to be part of the group doing it. Due to this little factoid, membership in the Klan remained relatively small throughout its first five years, only amounting to about 1,000 people in the Atlanta area. This changed in 1920 when Simmons hired a couple of marketing wizards to improve the Klan's image. While the whole "hey lets just hate and fear anything different" vibe was kept intact, the focus of the group was shifted from random lynchings to more community advocacy and charitable giving. With this new message in hand, the marketing wizards sent recruiters across the country who were paid a fee for every person they got to sign up. By 1924, membership in the organization had ballooned to 6 million, most of them middle class people living in the rapidly growing cities across the country. Especially large concentrations included Chicago, Detroit, Philadelphia, Indianapolis, Denver, and Portland.
Despite all of the parades, picnics, charity auctions, and cute baby contests, the Klan at its heart remained an organization about hating anything divergent from what its members saw as the norm. Though most members saw the existing political structure as being the best way to force their viewpoints on the rest of the world, few if any could honestly say they weren't aware of the more violent methods being used by some of their compatriots. Such actions by more radical members were conveniently forgotten. An unpleasant side effect to the preservation of an imaginary ideal. However, such issues couldn't be ignored forever. By 1925 the group was already beginning to crumble. From the beginning numerous politicians, including two presidents, denounced the group for its hate and vitriol. Local chapters were plagued by monetary fraud and infighting. Numerous members were arrested for violence, culminating in the conviction of the top Klansman in the Midwest for rape and murder. The idea of the Klan as just another fraternal organization collapsed and membership in the Klan collapsed with it. By 1930, only 30,000 members remained, many of them the worst of the worst.
The Ku Klux Klan never rose to such prominence again. Labeled as a group of hateful extremists, it was banished to the edges of American society. However, even in the fringe such things can be dangerous. When the Civil Rights Era began in the 1950's, the Klan launched a twenty year campaign of murders, bombings, and arson against the black population. Though membership in the revitalized organization likely never topped 50,000, many people were more than happy to look the other way or even secretly provide support. Many of those who carried out the violence weren't convicted until the 1990's or later. Others were never even arrested. Today the Ku Klux Klan is estimated to have around only 10,000 members spread across numerous independent chapters mostly based in the South and Midwest.