America has been awash in Prohibition-era nostalgia of late, with speakeasy-style bars, artisanal moonshine and “bootlegger balls” proliferating from New York to Cedar Rapids, Iowa, to Los Angeles, where revelers in period dress will pack that city’s 1930s Union Station to ring in the New Year.

But in her new book, “The War on Alcohol: Prohibition and the Rise of the American State” (W. W. Norton), the historian Lisa McGirr tells anything but a nostalgic story. The 18th Amendment, she argues, didn’t just give rise to vibrant night life and colorful, Hollywood-ready characters, like Isidor Einstein, New York’s celebrated “Prohibition Agent No. 1.” More enduringly, and tragically, it also radically expanded the federal government’s role in law enforcement, with consequences that can be seen in the crowded prisons of today.

In The New York Times Book Review, James A. Morone writes that the book “could have a major impact on how we read American political history.” In a recent email interview, Ms. McGirr, a professor at Harvard, discussed Prohibition’s political legacy, the surprising enforcement role of the Ku Klux Klan and the character from her story she’d most like to have a drink with. Below are excerpts from the conversation.

Q. The popular image of Prohibition is one of flappers, speakeasies and gangsters. What’s wrong with that picture?

A. Prohibition has been largely mined for its sensationalist entertainment value. Many chronicles have emphasized Prohibition’s vast inadequacies, and in some ways it was laughable. The government came nowhere near to eradicating the liquor traffic. But the very attempt had enormous and lasting consequences.

Q. You argue that Prohibition gave rise to today’s “penal state.” How did that happen?

A. By birthing a new national obsession with crime, Prohibition — and the violence that came with it — pushed the federal government in the direction of policing and surveillance. This was the moment that saw the first national crime commission, the birth of the Uniform Crime Reports, an expanded prison system and the establishment of the Federal Bureau of Narcotics. The F.B.I. also won expanded authority.

Q. You write that the ’20s roared only for a small slice of the population. What about everyone else?

A. Enforcement may have looked lighthearted in New York, especially for the middle class, who drank in protected speakeasies. But it was anything but a laughing matter for poor men and women in places like Virginia, North Carolina and Texas. Corruption up and down the enforcement chain smoothed the operation of large-scale criminal suppliers, but marginal violators, and those groups already identified with criminality, were not as lucky. They ended up filling police logs, jails and increasingly crowded federal and state prisons.

Q. You describe how the Ku Klux Klan helped enforce Prohibition in places like Williamson County, Ill., where federal authorities deputized its members to conduct sometimes deadly raids on distilleries, bars and private homes — taking particular aim at Italian immigrants. What made the Klan such an ally in the war on alcohol?

A. The Klan sold itself to white Protestant evangelicals as a law enforcement organization, winning droves of recruits with its promise to clamp down on bootlegging. There were plenty of Klansmen who imbibed, but that did not stop them from leveraging the law to target the drinking of the presumed enemies of white Protestant nationalism: immigrants, Catholics and African-Americans.

Q. It’s often thought that these groups moved decisively to the Democratic Party in 1932, when Franklin Roosevelt won the presidency in a landslide. But you argue that the breakthrough came in 1928, when Al Smith was the Democratic nominee. How important was Prohibition to that shift?

A. Before 1928, urban ethnic working-class political loyalties were divided between the Republican and the Democratic parties. Many recent immigrants had not yet built firm loyalties and were nonvoters. That changed when Al Smith ran for president, under the banner of opposition to Prohibition. The urban ethnic working class shifted heavily to the Democratic Party as a result. These men and women labeled Prohibition the tyranny of the Billy Sundays [Sunday was a prominent evangelical minister and Prohibition advocate] and identified it as an attack on their leisure and on Catholic religiosity. They also resented being classed as criminals. F.D.R. built on this new voting base, forging the New Deal coalition that lasted for much of the 20th century.

Q.What Prohibition-era character would you most like to have a near beer with?

A. A tough call, but it would probably be Clarence Darrow — though I am sure he would have refused to touch the much-hated near beer and gone in for some harder bootleg liquor. Darrow had opposed Prohibition before the amendment’s passage, and he became one of its most vocal opponents as it unfolded. He decried the violence and intolerance of men and women intent on enforcing the Volstead Act [the 1919 law carrying out Prohibition] and said essentially that they spawned an orgy of excessive zealotry and in some places outright terror. Given the story I uncovered in Williamson County, I think he was right.

Q. You draw a very straight line from the war on alcohol to the continuing war on drugs. Do you see room politically for that campaign to be repudiated as completely as Prohibition was?

A. Historians are not terribly good at predicting the future. Still, the consensus on the war on drugs is finally beginning to fracture, as it did for Prohibition over 75 years ago. Calls are ever louder for ending counterproductive penal approaches. I hope we embrace a turn toward a new New Deal for those who have borne the brunt of the drug war’s selective and often discriminatory enforcement.

Q. Is there any positive legacy of Prohibition that we might raise a glass to this New Year’s Eve?

A. Absolutely. I would raise a toast to the new mixed world of night life leisure that Prohibition spawned. Before that, public drinking was associated with the boisterous working-class male world of the saloon. Women who entered risked identification as prostitutes. Prohibition’s new semi-secretive world of leisure helped bring wider groups of women into these spaces, where they have remained ever since.