Beverly Gage is a professor of U.S. history at Yale.

How did this happen? Since 2016, it’s been a daily question in American politics. Sometimes it refers directly to President Trump, in all of his demagogic unpredictability. At other times, it’s more existential: How did we — America, of all places — end up in a state of endless squabbling and tribal hatreds? This past week, it mostly referred to the spectacle of an undisciplined president empowered to order the assassination of a foreign official without so much as a nod to Congress or apparent serious thought about the consequences of his actions.

The political scientist Andrew Bacevich has an explanation for all of it: American elites “squandered” the enormous power and promise bequeathed to them at the end of the Cold War, and this is what we got. While Trump may be an especially malevolent symptom of today’s ills, the disease has been at work for years, weakening what were once healthy tissues of our body politic and inflaming its internal tensions. “The Age of Illusions” is a wry and dark book aimed at dissecting decades-long trends and first principles rather than moment-to-moment crises. Bacevich is as merciless toward liberals who he says are guilty of “self-righteously posturing against Trump” as toward the president himself. Though completed well before the recent high-stakes brinkmanship with Iran, the book identifies the excessive warmaking power now concentrated in the presidency as one of the most alarming developments of the post-Cold War era, the sort of slow burn that can make a conflagration possible.

Best known for his critiques of the Iraq War, Bacevich has chosen in “The Age of Illusions” to write more broadly about the “contradictions” of what he describes as the post-Cold War consensus. A former Army officer and a professor emeritus at Boston University, Bacevich has never fit neatly into political categories. Once a true believer in Reaganism, he is now president of the Quincy Institute for Responsible Statecraft, a new foreign policy think tank devoted to ending “endless wars” and promoting military restraint. He writes for both the Nation and the American Conservative, and as he notes in the book, he voted for neither Trump nor Hillary Clinton. He calls himself a conservative but agreed to serve on Bernie Sanders’s 2016 foreign policy advisory team (which, according to Bacevich, never quite got off the ground).

Bacevich weaves snippets of his biography into “The Age of Illusions,” noting with some chagrin that he and Trump were born just 13 months apart, straight white men of a certain generation. For that generation, born into the Cold War, what it meant to be American often seemed simple: You hated communism, liked freedom and bought into the promise of consumer life. Bacevich notes that intellectuals, such as C. Wright Mills and Christopher Lasch, questioned the merits of this system, and one gets the sense that he would like “The Age of Illusions” to play a role similar to, say, Mills’s 1956 “The Power Elite,” even if, as he contends, the “practical impact” of such ideas never seemed to amount to much.

The Cold War mind-set, in Bacevich’s telling, came with certain disciplining features. Because the Soviet Union was always out there, doing its own thing, American statesmen could not be as reckless as they might otherwise have been, and American citizens were forced to live up to some modicum of their own oft-stated virtues. But the Cold War also constituted a “tragedy of towering proportions,” Bacevich writes, 40-plus years of “folly and waste,” all to create the greatest buildup of lethal force in human history. The proper response, when it came to an end, would have been “reflection, remorse, repentance, even restitution.” Instead, an “intoxicated elite” rushed ahead into the 21st century, giddy with its own power and wealth, sure that it could now, at last, remake the world in America’s image.

What emerged from this enthusiasm, Bacevich argues, was a dreary new post-Cold War consensus, built around a commitment to neoliberal economics at home and abroad, backed by American military supremacy and an increasingly powerful White House. Technology was supposed to bring it all together, as smart bombs and drones replaced messy human warfare. And everyone was supposed to win: America, the world, the poor, the rich, the cause of human freedom. Bacevich likens the late 1990s to the moment when Dorothy and her “Wizard of Oz” companions arrive at the yellow brick road, convinced that their troubles have come to an end. We all know how well that turned out.

Bacevich’s emphasis on the past 30 years as a period of consensus departs from an arguably more popular and widespread narrative, in which American politics during that time fell victim to polarization and hyper-partisanship, and to entrenched conflicts over race, culture and civil rights. “The Age of Illusions” does not devote much time to differences of opinion within the so-called elite, though Bacevich defines that category widely, including NPR listeners and Washington Post subscribers (that may be you, dear reader), along with “Wall Street, Silicon Valley, the Israel lobby, the National Rifle Association, the national security apparatus, and megadonors like the Koch brothers.” He sees today’s more meaningful divide as one of class, with poor and working-class Americans fighting America’s wars as supposed volunteers, while the “disturbingly inbred and self-perpetuating political elite” who sent them off to battle remains insulated from the consequences of its own decisions.

Though no fan of the current president, Bacevich reserves his greatest ire for establishment types who spend their time griping about Trump while refusing to examine their own role in creating the inequalities and foreign policy disasters of recent decades. The Trump presidency is a result of their failed promises, in his view: on jobs and economic security, on Afghanistan and Iraq. Trump himself gets off relatively easy, as a “noxious and venal” but largely ineffective “blowhard,” unable to put his ideas into action. Despite kids in cages and a full-blown impeachment crisis, “most of the horrors predicted by Trump’s critics have not come to pass,” Bacevich insists, “at least not as of this writing.”

That last caveat may yet prove to be crucial, of course. As Bacevich notes, even the most astute Cold Warriors missed the impending signs of that system’s collapse, convinced that what had existed for 40 years would continue forever. Today, there may be more awareness that the old ways are crumbling, but nobody seems to know what will come next. Bacevich, for one, views Trump as a “transitory figure,” useful for identifying certain contradictions and illusions within the established order, but still operating largely within its parameters.

Even Trump’s decision to order the assassination of Iranian Maj. Gen. Qasem Soleimani looks to Bacevich less like a radical break with the past than an extension of the imperial ambitions and concentrated presidential power that have dominated American foreign policy since the end of the Cold War. “The game remains unchanged,” he wrote in the Los Angeles Times, at a moment when many other pundits were decrying Trump’s actions as uniquely reckless and ill-conceived. “It’s a game that the United States has been playing — and losing — for close to a generation.”

The Age of Illusions

How America Squandered Its Cold War Victory

By Andrew J. Bacevich

Metropolitan. 236 pp. $27