RUTHFULLY YOURS

That, at any rate, is how it ought to be. But instead of expanding our minds and hearts, history is increasingly used to narrow them. Instead of helping us to deepen ourselves and take a mature and complex view of the past, history is increasingly employed as a simple bludgeon, which picks its targets mechanically—often based on little more than a popular cliché—and strikes.

The best example may be the evergreen argumentum ad Hitlerum, in which every evil from bigotry and militarism to vegetarianism and appreciation of Wagner’s operas is referred to the transcendentally evil standard of Nazism. The detention centers on America’s southern border should be called “concentration camps,” according to Rep. Alexandria Ocasio-Cortez. When questioned, the young, irrepressible Democrat advised Americans: “This is an opportunity for us to talk about how we learn from our history.” But that history isn’t ours. By invoking such an emotionally laden term, she was playing on a potent theme, but in a way that underscored the limited range of her historical reference, as well as the public’s.

A more disturbing example is the pell-mell rush to pass judgment against heroes of the past and tear down or rename the monuments to them—including George Washington, Thomas Jefferson and Woodrow Wilson. Are we really so faint of heart that we can no longer bear to allow the honoring of great men of the past who fail in some respects to meet our current specifications?

It’s true that all three men held either slaves or racist beliefs. Does that exhaust everything we need to know about them? Ought it to outweigh the value of everything else they did? For those who say yes, the transformation of history into a weapon depends upon a brutal simplification of the historical record. Such is the approach of the New York Times’s audacious “1619 Project,” which argues “that nearly everything that has made America exceptional grew out of slavery.”

A genuinely historical approach would acknowledge, even insist on recognizing, that Washington owned slaves. It would go on to consider that fact from the larger perspective of a long, important and consequential life. It would weigh Washington’s beliefs and actions carefully in the context of their time, and would take into account his decision to free his slaves at the time of his death.

That kind of respectful detail and complexity seems to be leaving with yesterday’s fashions. Instead, we get patent idiocy. The San Francisco School Board voted in June to spend up to $600,000 to paint over a high school’s mural depicting the life of Washington. Two weeks ago it voted to cover up the artwork instead—a compromise. The 1,600-square-foot mural was painted in 1935 by a communist who sought to include Washington’s ownership of slaves as part of a complex portrait of him. But the school board decided that complexity was too disturbing to teenagers, and that the mural was racist and degrading in its depiction of black and Native American people. Better to have plain white walls—or morality tales depicting “the heroism of people of color in America,” as is the new plan—than to tell a complicated story about an American hero.

The weaponizing of history corresponds invariably with a remarkable hostility to history. Its practitioners are content to slice a single fact out of a web of details, then repeat that fact with the stubbornness of protesters who have memorized a chant.

This aggressive historical simplification is at the core of the cult of intersectionality, which now rules American college campuses. The language of unchallengeable collective grievance relies on history for its authority. Notice how concepts such as “historically underrepresented” and “historically marginalized” are used to certify groups that deserve to be favored automatically in the present.

The condition of any particular person doesn’t have a reliable relationship to that aggregate group victimization. But the key move is to draw on the authority of history to construct unanswerable arguments in every dispute, and always refer individual cases to the invincible aggregate norm.

Why study the past? Today, the point is too often to gain ever better weapons to use in present battles, ever more unanswerable supports for our grievances. This argument from history is potent precisely because it relies on conclusions drawn from data that are no longer ready at hand. It all comes out of a black box called History.

But that cannot last forever. Once history becomes a club, it quickly loses its credibility as history. The grossly exaggerated claims of the Times’s “1619 Project” are likely to bring on just such discredit.

Done right, history rescues precious memories from the darkness into which they would otherwise disappear, forging a sense of continuity with the past. If we care about history, we now must rescue it from its crudest instrumentalizers and insist upon its richness and complexity. Our task is to recover the humane insight of Herbert Butterfield, who taught that the historian should be a “recording angel” rather than a “hanging judge”—let alone a summary executioner.

Mr. McClay is a professor at the University of Oklahoma and author of “Land of Hope: An Invitation to the Great American Story.”