‘Holy f---. This can’t last. This is literally insane.” This salty quote, coming about midway through Peter Bergen’s rollicking account of President Trump’s foreign policy, is how an unnamed senior official describes the response of the two original adults in the room, Defense Secretary Jim Mattis and Secretary of State Rex Tillerson, to one of many presidential decisions they felt unwise and uninformed — in this instance, the blithe approval of a Saudi-led blockade of Qatar in 2017. Soon enough, the government careers of both of these experienced and proud men would meet an unceremonious end. But their sentiments succinctly capture the way most of official Washington — and much of the world — thinks about U.S. foreign policy under Trump.

From the moment Trump strutted into the Oval Office, we have been buried by an avalanche of jaw-dropping revelations about what happens when an unhinged, cynical and impulsive commander in chief bumps up against professionalism, decency and the rule of law. So when opening a new book promising still more inside stories of Trump’s foreign policy, it is hard to expect an author to say anything new — especially when the book was written before the impeachment drama started. Perhaps the best one can hope for is something that helps put this craziness in perspective and lays out the stakes for the future.

Bergen’s “Trump and His Generals” meets the test. A respected national security analyst at New America and CNN, Bergen provides a deeply informed study, written with clarity and flair. Reflecting fresh research and nearly 100 interviews with some key players, his retelling of Trump’s foreign policy skillfully synthesizes what’s already known and adds gossipy tidbits. Although it doesn’t change the fundamental story line and may not create breaking news, it is the best single account of Trump’s foreign policy to date.

The narrative arc of Bergen’s tale is familiar, showing how the relationship between the president and the military brass who initially staffed his administration — retired Marine generals Mattis and John Kelly, and Army Lt. Gen. H.R. McMaster — went down in flames. Bergen traces the ways Trump’s fanboy fascination with “his” generals transformed into deep frustration with their caution and ethical code.

Trump initially saw these men as cartoon characters — killers, he liked to call them, as though they were schooled exclusively in the ways of John Rambo. At first Trump crowed about “Mad Dog” Mattis like a mafia boss brags about his hired muscle, but he soon dismissed him as a “Little Baby Kitten” and mocked him as overrated. McMaster suffered an even more humiliating fate, as Trump quickly tired of his professorial briefings and disparaged his civilian suit because he looked “like a beer salesman.”

As Bergen tells it, Mattis and McMaster were motivated to work for Trump by their perceptions of the Obama administration’s failures, especially in the Middle East. They thought Obama had squandered American leadership by not enforcing the red line in Syria, ceding ground to Moscow, withdrawing from Iraq and being too timid in the fight against the Islamic State.

Ironically, they ended up serving a president who wanted out of the region far more than Obama ever did. Instead of drastically altering the U.S. approach toward the Islamic State, Bergen explains, they followed a strategy that was essentially the same. And they found themselves doing everything possible to save Obama policies (like the Iran nuclear agreement) that Trump was determined to destroy.

Bergen recounts how Mattis used all the tricks of the trade to thwart some of Trump’s most dangerous instincts, slow-rolling requests for military options or ignoring him altogether. “We have to make sure reason trumps impulse,” he quotes Mattis as saying. This seems laudatory when considering the nature of Trump’s requests: from telling his top national security officials that Seoul’s 10 million residents had to relocate, to questioning the U.S. commitment to NATO, to threatening to withdraw forces from South Korea or Afghanistan.

Yet this raises important questions for the future of civil-military relations. Do we want it to become routine for military leaders to ignore requests by a president and other civilian leaders? Would we feel the same way if this was how a future president — say, Elizabeth Warren or Nikki Haley — were treated? If it is okay, then under what circumstances? Bergen doesn’t say, and one wonders what he thinks.

Moreover, when reconsidering these tales of how Mattis and company worked to redirect Trump’s impulses, one must also conclude that they had the perverse effect of enabling him. If the adults were in the room, we could rest assured that things would not get out of hand, thus denying the reality that Trump would never change. As long as they were around, we could be seduced into thinking that things could be normal — as Mattis would say, he was the “Secretary of Reassurance.”

But now they’re gone. And as Trump takes a sledgehammer to so much they tried to preserve, the silence of Mattis, Kelly and McMaster is deafening. One finishes Bergen’s book with a sinking feeling that things are going to get worse — and the military’s role will be stressed in ways not seen in decades.

Bergen illustrates some of the ways Trump has been good to the Pentagon, showering it with bigger budgets and granting field commanders greater leeway. Yet recently things have taken an ugly turn, with the president surprising military leaders by abandoning the Kurds in northern Syria. Even more concerning, in echoes of McCarthy-era attacks on the Army, Trump and his family have promoted conspiracies questioning the loyalty of a senior military officer who serves in the White House. And Trump has championed the cause of Special Forces troops accused of war crimes, upending the military criminal justice system to shield some of them from punishment and leading to the firing of Navy Secretary Richard Spencer. Writing in The Washington Post, Spencer said that in intervening on behalf of Navy SEAL Edward Gallagher, the president showed “very little understanding of what it means to be in the military, to fight ethically or to be governed by a uniform set of rules and practices.”

Depending on the outcome of impeachment and the 2020 election, we may be in the kind of crisis where the military leadership — and all of us — will face an even more uncomfortable and dangerous situation. If the president survives and wins reelection, he will be empowered and emboldened to sow chaos. If he loses, we should expect him to triple-down on what he’s already saying: blaming enemies and traitors who hate America and who are trying to steal the presidency in a coup to destroy the country. While it may seem far-fetched, we should carefully consider the circumstances when Trump once again looks toward “his” generals — and ask, what will they do? As Bergen concludes his book, the choice is clear: They either “go along for the ride or resign.”

Trump and His Generals

The Cost of Chaos

By Peter Bergen

Penguin Press. 386 pp. $30