If you only read F. Scott Fitzgerald, you might get the impression that everyone during the 1920s flouted Prohibition and got away with it. But while it’s true that a small portion of white, middle- and upper-class urbanites lived that way, the Roaring Twenties was also the decade in which the newly revived Ku Klux Klan expanded across the country under the guise of enforcing Prohibition.

The Klan’s main targets were immigrants from southern and eastern Europe, especially Catholic ones. Prohibition advocates had already linked them with drinking and criminality, and for these people, the era was a time of raids, violence and terror.

From the beginning, Prohibition was tied up with anti-immigrant and anti-Catholic biases. Many of its advocates were white, Anglo-Saxon Protestants who thought only people like them could be “real Americans.” They believed the country was under siege by Catholic immigrants from countries like Italy, and that these people threatened the U.S. with their foreign drinking habits and saloons.

“It was really a battle for cultural supremacy in a country that was changing,” says Thomas R. Pegram, a history professor at Loyola University Maryland and the author of One Hundred Percent American: The Rebirth and Decline of the Ku Klux Klan in the 1920s. “Prohibition became a way in which that could be enforced in local communities.”

The two major organizations that lobbied for national Prohibition—the Women’s Christian Temperance Union and men’s Anti-Saloon League—blamed Catholic immigrants in the 1910s for the “saloon culture” they felt was plaguing the nation. The League even argued that the U.S. needed to pass a national ban before its demographics changed too much.

“They believed that if they didn’t push for a constitutional prohibition before the 1920 census, and before congressional districts were reapportioned based on population increase, that they wouldn’t be able to get prohibition because there’d be too many acculturated new citizens who had been immigrants in the previous two decades who would prevent that,” Pegram says.

And indeed, the U.S. did pass it before then. The states ratified the 18th Amendment on January 16th, 1919, and it took effect in 1920. During that decade, the criminal justice system expanded as police disproportionately arrested people who were immigrants, black, poor and working-class. But there were also plenty of Prohibition-supporting white Protestants who thought the law wasn’t doing enough to stop the bootleggers they read about in the tabloids.

That’s where the KKK stepped in. It sold itself to those people as a law enforcement organization that could do what the government couldn’t—put a stop to the Catholic immigrants supposedly violating the law.

“The reason that the Klan was able to basically bring millions of Protestant white evangelical Americans to its ranks in the 1920s is definitely related to the passage of Prohibition and the 18th Amendment,” says Lisa McGirr, a history professor at Harvard University and author of The War on Alcohol: Prohibition and the Rise of the American State.

“Prohibition provided the Klan essentially a kind of new mandate for its anti-Catholic, anti-immigrant, white Protestant nationalist mission,” she says. “The Klan often gained a foothold in local communities in the 1920s by arguing that it would clean up communities, it would get rid of bootleggers and moonshiners.”

The Klan began raiding Catholic immigrants’ homes, burning down their businesses and planting evidence to use against them. The Klan wasn’t necessarily trying to put these people behind bars—though many immigrants did end up in jail—but rather to terrorize these communities. The fact that Klansmen sometimes seized alcohol only to drink it themselves was a clear sign that their raids weren’t just about enforcing Prohibition.

The Klan at this time was actually in its second incarnation. The original version of the Klan died during Reconstruction because the government shut it down. In 1915, it experienced a resurgence with the film The Birth of a Nation, which romanticized and popularized the terrorist organization. During the ‘20s, the Klan—along with its auxiliary “Women of the Ku Klux Klan” and three KKK youth groups—spread across the north and south by arguing that Catholics and immigrants were breaking Prohibition, and only a vigilante police group like the Klan could put a stop to it.

“I would say the Klan of the 1920s was unusual in that its primary focus was on Catholics and eastern and southern European immigrants,” Pegram says. “That’s partly because African Americans had already been segregated and deprived of the vote, and in official life kind of marginalized in the United States.” Even so, the Klan did target black Americans and their nightclubs in the name of “Prohibition enforcement” (government police targeted them too).

Between 1920 and 1925, the Klan’s membership grew to some two to five million, and there was a lot of overlap between these new members and those who supported Prohibition. “The WCTU [Women’s Christian Temperance Union], the Anti-Saloon League and the Klan were definitely not one in the same but there was a lot of overlap in their goals, there was a lot of overlap in their ideology, and there was a tremendous amount of overlap in terms of their support for their activities in the 1920s,” McGirr says.

“Remember that those men and women who had worked so hard to pass the amendment were deeply disconcerted by the continued overwhelming violations,” she says. “One of the primary ways that women came into the Klan in a place like Indiana, where the Klan had a lot of power, was through the WCTU.” In Williamson County, Illinois, one pastor “admitted that while he had ‘been accustomed to working through the Anti-Saloon League,’ now the hooded order provided a more militant vehicle,’” McGirr writes in her book.

Prohibition lasted less than 15 years, but it left behind a large legacy. When it ended in 1933, the U.S. government had a more powerful FBI and a lot more prisons. As for the Klan, the upheaval and chaos that it created during the 1920s eroded its support in the ‘30s. Yet the organization’s history didn’t end there.

The Klan experienced its third resurgence during the civil rights movement and saw an uptick in activity and increased national attention after the 2016 presidential election. The Southern Poverty Law Center estimates that the Klan, a century after Prohibition, has between 5,000 and 8,000 members.