It is a truth universally acknowledged that revolutionary sentiment is inherently expansionist and, if unimpeded, always winds up devouring its earlier partisans.

As we write, the social-justice chain Starbucks, famous for hectoring its customers about progressive politics and serving expensive burnt coffee, is being boycotted because a store manager in Philadelphia had two black loiterers arrested and removed by the police. Kevin Johnson, the company’s spineless ceo, sized up this brewing mess and emitted a pathetic damage-control bulletin, calling the manager’s act “reprehensible” and vowing to “be better” for “customers who come to us for a sense of community every day.” (And here we thought that people went to Starbucks for coffee. Nothing so pedestrian, it turns out: it’s a “sense of community” they crave behind that Grande Mocha Frappuccino.) On May 29, in a gesture of ritual expiation, the wretched chain will close all 8,000 of its stores for an orgy of liberal guilt and “racial bias training.” Pathetic.

No major New York museum is more politically correct than the Brooklyn Museum.

To compare large things with small (or perhaps it’s the other way around), consider the comical temper tantrum now swirling about the Brooklyn Museum. No major New York museum is more politically correct. Located in the outer boroughs, it has to do something to attract attention. So it hosts pornographic exhibitions like “Sensation,” which drew in the crowds back in 1999, and makes a big deal of feminist embarrassments like Judy Chicago’s The Dinner Party (“a milestone in twentieth-century art,” its press material said). In 2016, the museum bestowed upon Angela Davis its Sackler Center First Award, “honoring women who are first in their fields.” It’s hard to cavil with that description. Ms. Davis, the former Black Panther, is surely the first recipient of the Soviet Union’s Lenin Peace Prize, two-time vice-presidential candidate on the Communist Party ticket with Gus Hall, and winner of a spot on the fbi’s Ten Most Wanted List (a fugitive wanted for material accessory to murder) to have parlayed that resume into a tenured professorship at the University of—can you guess? yes! at the University of California.

All of which is to say that the Brooklyn Museum, especially under its newish director Anne Pasternak, is a model of political correctness. Ms. Pasternak cut her teeth organizing exhibitions like “The Abortion Project” at alternative art spaces. At the Brooklyn Museum she has championed curators who take an “anticolonial approach to curating” with exhibitions like “The Legacy of Lynching: Confronting Racial Terror in America” and “We Wanted a Revolution: Black Radical Women, 1965–85.”

If you think all that rubbish purchased Anne Pasternak or the Brooklyn Museum some street cred with the Left, think again. No sooner had the museum announced that its new curator of African Art would be Kristen Windmuller-Luna than the protests began. For Ms. Windmuller-Luna, you see, is that diminished thing, a white woman. Appointing her as the curator of African Art is an example of what social justice warriors call “cultural appropriation” that perpetuates the Brooklyn Museum’s legacy of “colonial” attitudes.

Anyway, that’s what two open letters have angrily alleged. These documents make it clear that Ms. Windmuller-Luna’s actual qualifications (such as they are) have nothing to do with the case. At stake is not competence or expertise but identity politics. Martin Luther King, Jr. urged us to look beyond the color of our skin to the content of our character. That attitude has long ago been enrolled in the index prohibitorum of “racist” back-pedaling.

According to these remarkable attacks, the appointment of Ms. Windmuller-Luna constitutes a “curatorial crisis” for the museum. All is not lost, however, for the alleged “crisis” offers the museum an “Opportunity to Decolonize.” That’s right. The petition writers do not just want a more swarthy curator to replace Ms. Windmuller-Luna. They want to challenge “the pervasive structures of white supremacy in the art field.” They want a “structural response” (“structure,” like “crisis,” is a favorite word), and they therefore call for the creation of a “Decolonization Commission” to “redress ongoing legacies of oppression, especially when it comes to the status of African art and culture.”

Those “legacies of oppression” encompass much more than the status of African art and culture, however, so the process of “decolonization” must start “with the acknowledgment that the buildings sit on stolen indigenous land, that they contain thousands of objects expropriated from people of color around the world, and that the institution is governed by a group of majority-white members of the 1 percent actively involved in the dynamics of racialized dispossession and displacement in Brooklyn.” Oh dear.

And that’s just the start, you see, because, as you may have already guessed, “decolonization is never a finished process.”

The institutions that signed these letters—twelve for the first, which grew to nineteen for the second communication—are a smorgasbord of loopy left-wing groups. “Decolonize This Place” and the “Brooklyn Anti-Gentrification Network (ban)” are signatories, for example, along with “Flower Lovers Against Corruption (flac),” “Dancing for Justice,” and “Occupy Museums.” “We belong to communities that are . . . engaged in day-to-day struggles against settler-colonialism, white supremacy, patriarchal violence, police terror, mass incarceration, population displacement, deportation, economic precarity, and climate disaster.”

Has anything been left out? Is there any left-wing cause that has not been fitted with its demand? What about Israel? No self-respecting left-wing activist movement can proceed without at least ritual bashing of Israel. Sure enough, among the Brooklyn Museum’s sins is having “provided cover for pro-Israeli artwashing [sic] in the exhibition of the lavishly- funded This Place photography show.” Presumably, the fact that the exhibition was “lavishly funded” deepens the heinousness of the tort. The authors come back to Israel in their demand that the museum demonstrate its “institutional commitment” to the “Boycott, Divestment, and Sanctions (bds) movement” against Israel, especially as it implicates Jews in Brooklyn who may be involved in the settler movement in the West Bank.

Like any such initiative worth its salt, these “Open Letters” end with a long list of demands, which are even more comical than the narrative that prefaces it.

They want an “inventory of colonial-era objects of both African and Indigenous people with a view to settling the long-pursued claims of reparations and repatriation” and also “the replacement of Board president David Berliner and other trustees who are real estate tycoons with a broad cross-section of artists and community organizers.” Getting rid of people who actually support the museum financially is going to make another key demand easy to achieve. We mean the effort to start a “de-gentrification initiative to examine and mitigate the museum’s role in boosting land value and rents in the borough.” We are certain that, should these lunatics get their way, “de-gentrification” and plummeting land values would be the rapid result.

Anne Pasternak is an ally in their silliness, not an opponent.

It goes without saying that such childish antics are simply business as usual in the self-important fever swamps of the Left. In terms of substance, they are merely part of the toxic blanket of identity politics that swaddles our educational and cultural institutions today. But not only do they afford a sterling instance of how the Left devours itself—Anne Pasternak is an ally in their silliness, not an opponent—they also illustrate a hard truth that Winston Churchill articulated in 1937 when he appeared before the Peel Commission on creating a Jewish state among the Palestinian Arabs. Churchill was in favor of such a state, for reasons that once counted as common-sense wisdom but which can now hardly be uttered. “I do not admit that the dog in the manger has the final right to the manger, even though he may have lain there for a very long time,” Churchill said of the Palestinians. By way of explanation, he went on to say,

I do not admit, for instance, that a great wrong has been done to the Red Indians of America, or the black people of Australia. I do not admit that a wrong has been done to those people by the fact that a stronger race, a higher grade race, or, at any rate, a more worldly-wise race, to put it that way, has come in and taken their place. I do not admit it. I do not think the Red Indians had any right to say, “The American Continent belongs to us and we are not going to have any of these European settlers coming in here.” They had not the right, nor had they the power.

We might express the sentiment differently, but it speaks to a commonplace reality, or what used to be one before political correctness clouded our faculty for apprehending difficult truths. The fact that such rough truth sends academic and cultural partisans of identity politics into a frothing tizzy is just one more reason that they deserve our pity as well as our contempt, but, above all, our vigilance.