Imagine making a documentary about one of the 20th century’s leading opponents of the Ku Klux Klan — without ever talking about the evil of the KKK itself.

If that sounds like malpractice, consider PBS’s new documentary on the life of William F. Buckley Jr.

“The Incomparable Mr. Buckley,” the latest installment in the “American Masters” series, has much to say about anti-Communism but never reckons with the murderous reality of Communism itself.

In failing to do so, producer and director Barak Goodman unintentionally reminds his viewers of why Buckley was needed in the first place — and why he still is.

Never mind that Buckley died in 2008, and next year marks the centenary of his birth.

The liberals who already reigned in America’s universities when Buckley was a Yale student in the late 1940s have not learned any lessons in the decades since then.

Faculty and administrators still will not speak frankly about evils emanating from the left end of the political spectrum, from Communism to the many violent groups that claim to act in the name of anti-colonialism.

The PBS documentary gets Buckley’s resume right but understands little of its significance.

In 1951 Buckley published his first book, “God and Man at Yale.”

Four years later, when he was not quite 30 years old, Buckley launched National Review, which became the all-but-official publication of the nascent conservative movement.

He had a hand in the creation of other institutions, too, such as the right’s student activist arm, Young Americans for Freedom.

Barry Goldwater and Ronald Reagan drew political support and intellectual sustenance from the movement Buckley built.

And after Goldwater’s crushing defeat in the 1964 presidential election, Buckley restored conservatives’ spirits with his run for mayor of New York City the following year.

“The Incomparable Mr. Buckley” is right to suggest that although WFB’s mayoral campaign never had a shot at winning — Buckley joked that if he won, he’d demand a recount — it taught conservatives how to mobilize urban Catholics and voters fed up with escalating crime.

The Nixon coalition, which would win the White House in 1968 and 1972, was the Buckley coalition first.

His mayoral run made him a media sensation and led to a career in television, on top of the several careers he already had as an author, editor, lecturer and movement-maker.

He even started his own highly successful interview show, “Firing Line,” which ran for more than three decades, mostly on PBS affiliates, starting in 1966.

“The Incomparable Mr. Buckley” tantalizes viewers with clips of WFB’s exchanges with “Firing Line” guests such as Norman Mailer and Allen Ginsberg.

But the documentary is reluctant to let Buckley speak for himself; voiceovers from historians offering their own spin break in after only a few words from the subject himself.

The filmmakers prefer to highlight defeats and embarrassments: the debate Buckley lost to James Baldwin at Cambridge University in 1965 on the resolution “The American Dream is at the expense of the American Negro” and WFB’s explosion on live TV, while covering the 1968 Democratic National Convention, when Gore Vidal taunted him as a “pro- or crypto-Nazi.”

Buckley, losing his composure for once, retorted by calling Vidal a “queer” and saying he’d “sock” him in the face — “and you’ll stay plastered!” — if he kept up the abuse.

Vidal delighted in getting this rise out of Buckley and thought it made great television, but the conservative was mortified.

The trouble with “The Incomparable Mr. Buckley,” though, isn’t that it showcases such episodes but that it finds its subject incomprehensible at the most important level — the meaning of his life’s work.

When Goodman isn’t presenting Buckley as a figure fit for “Lifestyles of the Rich and Famous,” he and the historians he’s enlisted press the thesis that Buckley was an irresponsible elitist who dabbled with populist forces he could not control.

The documentary ends with scenes of Donald Trump and the Jan. 6, 2021, riot at the U.S. Capitol.

But it’s elite liberals, not Buckley, who created the opening for Trump.

Buckley’s institutions, notably National Review, opposed Trump — yet their opposition wasn’t enough to offset demand for Trump from voters whom liberals had alienated.

By failing to learn the lessons Buckley tried all his life to teach, and refusing to moderate their left-wing prejudices in light of an articulate conservative critique, liberals in politics, media and the academy guaranteed the rise of populism.

From the Cold War to crime in the cities, they blamed America for every problem.

Thirty-five years after the fall of the Berlin Wall, liberals like those behind “The Incomparable Mr. Buckley” persist in treating Communism as a footnote to McCarthyism.

They have their history, and their view of Buckley, upside down.

Daniel McCarthy is the editor of Modern Age: A Conservative Review. To read more by Daniel McCarthy, visit www.creators.com