In the 1990s, Trump made headlines for lobbying the New York State legislature to ban disabled veterans from working as street vendors around Trump Tower. “Do we allow Fifth Avenue, one of the world’s finest and most luxurious shopping districts, to be turned into an outdoor flea market, clogging and seriously downgrading the area?” he wrote to a state assemblyman. Thirteen years later, he appealed again to then-Mayor Michael Bloomberg. “Whether they are veterans or not,” he wrote, “[the vendors] should not be allowed to sell on this most important and prestigious shopping street.”

At times, his remarks on veterans and military service have veered into outright mockery. In a 1997 interview with Howard Stern, Trump likened his determination to avoid sexually transmitted infections to serving in combat. His sex life in the 1980s was “my personal Vietnam,” he said. “I feel like a great and very brave soldier.” Last summer, he declared that Senator John McCain—a former Navy pilot who was held prisoner for five and a half years and tortured by the North Vietnamese—was “not a war hero.” His reasoning: “I like people that weren’t captured.”

Meanwhile, when Trump has weighed in on national security questions, his remarks often reveal either ignorance or disdain for military expertise and the codes of conduct that govern the armed forces. “I know more about ISIS than the generals do. Believe me,” he boasted in one speech, adding, "I’ve had a lot of wars of my own. I’m really good at war." His foreign policy prescriptions include proposals to “bomb the shit out of ISIS,” to “take out” the families of ISIS members and to torture terrorism suspects. (“Would I approve waterboarding? You bet your ass I would,” he told one crowd. “And you know what? If it doesn't work, they deserve it anyway, for what they're doing.”) When it was pointed out that soldiers couldn’t legally carry out those last two actions, Trump was unconcerned. "They're not going to refuse me. Believe me.” (He walked back that last statement the next day.) The Geneva Conventions, he recently observed, have made American soldiers “afraid to fight.”

Trump’s pronouncements on foreign policy, combined with his years of broadsides, have set off a very real fear within military circles about what might happen were he to become president. In the last two months, I spoke with dozens of people in the national security realm—current and retired officers, veterans of the wars in Iraq and Afghanistan, and former White House, State Department, Pentagon and CIA officials. The words they used to describe their mood: Terrified. Shocked. Appalled. Never before, they say, has a candidate gotten so close to the White House with such little respect for the military.

“He completely misunderstands the military profession that he would head if he were the president,” said Robert Killebrew, a retired colonel who served in the Army for more than 30 years. Others were less polite. In a pair of ads produced by the American Future Fund, a retired Special Forces commander named Michael Waltz calls Trump a draft-dodger who “hasn’t served this country a day in his life,” and a Vietnam veteran, Tom Hanton, says that Trump’s quip about POWs was “the most infuriating comment I think I’ve heard from a politician in my entire life.” One former Marine infantry officer described Trump to me as a “fake-bake-ing chicken hawk” whose “knowledge of the Middle East could be trumped (sorry) by your average Georgetown sophomore.”

Trump’s chosen foreign policy advisers—which include a 2009 college graduate who touted his experience in the Model U.N. on his online résumé and another who used Kanye West lyrics to make arguments on his foreign policy blog—have only stoked these anxieties. “Weirdo nobodies,” was how one military historian characterized them to me. “They’re probably the least qualified group of foreign policy and national security advisers I’ve ever seen or even heard of,” said Richard Kohn, an expert in civil-military relations and retired professor at the University of North Carolina. A source with firsthand knowledge told me that Trump’s campaign pursued retired General David Petraeus, Middle East scholar Daniel Pipes and former U.N. Ambassador John Bolton, but all three men declined. In a TV appearance not long afterward, Trump said he wouldn’t hesitate to replace the members of his military team if they didn’t agree with him. (Trump’s campaign didn’t respond to multiple requests for comment.)

For even the savviest of presidents, the relationship between a commander in chief and his military is famously fraught, an intricate dance of egos and agendas, worldviews and bureaucracies. A President Trump, however, could usher in a clash of historic proportions. “If you take the man at his word,” said Michael Breen, the president of the Truman National Security Project and a decorated former Army officer, “we have a presidential candidate who seems to have committed himself to triggering what would probably be the greatest crisis in civil-military relations since the American Civil War.”

Powell had no intention of letting this happen. The four-star general viewed the new president and his team as “children,” as diplomat Richard Holbrooke later recalled. Powell was a charming operator with an inspiring life story who had spent most of his nearly 35-year career in Washington and knew how to work the levers of power. (“You show me a general in Washington who ain’t political and I’ll show you a guy who ain’t going to get promoted again, and probably should not be a general in the first place,” he once said.) Shortly before Clinton’s inauguration, he told a class of Naval Academy midshipmen that if they found Clinton’s policy “completely unacceptable and it strikes to the heart of your moral beliefs, then I think you have to resign.”

Five days after Clinton’s inauguration, Clinton met with Powell and the rest of the Joint Chiefs in the Roosevelt Room. “[Homosexuality] is a problem for us,” Powell informed the new president, according to a notetaker. One by one, the rest of the generals flatly opposed the president’s plan. Furthermore, as former White House aide George Stephanopoulos wrote in his memoir, the Joint Chiefs had already lined up congressional support for their position: “Their message was clear: Keeping this promise will cost you the military. Fight us, and you’ll lose—and it won’t be pretty.”

And that’s essentially what happened. Powell did little to quash rumors that the entire Joint Chiefs would resign if Clinton went through with the policy. During the next nine months, the battle over the issue cost Clinton dearly. Eventually, he agreed to Powell’s proposed compromise, which we now know as “Don’t Ask, Don’t Tell.” “The new president had been rolled,” wrote historian Matthew Moten in his book Presidents and Their Generals.

The Bush administration, meanwhile, provided a disturbing preview of the damage that can be done when a civilian leadership pushes the armed services to adopt policies of dubious legality. Donald Rumsfeld, Bush’s defense secretary, was regarded as uninterested in the military’s advice and dismissive of the Joint Chiefs. After September 11, 2001, Rumsfeld authorized the use of “enhanced interrogation techniques”—practices that exposed the U.S. to charges of violating the Geneva Conventions and the War Crimes Act. The decision set off years of turmoil within the armed services. Retired Colonel Lawrence Wilkerson, the former chief of staff to Colin Powell, who was by then secretary of state, said that there were numerous instances of individuals in the chain of command sitting on the order to introduce abusive—and, some thought, illegal—interrogation methods in December 2002. “There was never a collective agreement to do the things that Rumsfeld told people to do in that memo,” Wilkerson recently told me. “There were certain leaders that passed on and gave orders based on those documents, but there are lots and lots of people who walked away from it and refused to do it.”

Alberto Mora, the Navy's general counsel, memorably fought to ban any use of enhanced interrogation techniques, which he believed were torture. Mark Fallon, the deputy commander of the Pentagon's Criminal Investigation Task Force, similarly argued that allowing his agents to witness the acts described in Rumsfeld's order, let alone implement them, would be disastrous. "Either they'll watch the law being broken, or, as sworn law enforcement officers, seeing laws being broken, they may try to arrest the military interrogators,” he told his boss, CITF Commander Colonel Brittain Mallow, as recounted in ex-FBI agent Ali Soufan’s book, The Black Banners. “Nothing good can come of this."

Retired military officers told me that the effect of this kind of internal dissension can be crippling. "You have usually an enormous decrease in morale because you sense that there's nothing you can do about it," Wilkerson told me. "Your values are being disrupted; you feel powerless." Andrew Bacevich, a retired Army colonel and military historian, said that "any policy that is inconsistently applied creates confusion and consternation in a hierarchical organization."

On some occasions, the military has been able to use the tools at its disposal—the press, Congress, calculated leaks, endless bureaucratic maneuvering—to reshape a president’s entire policy. President Barack Obama, for instance, arrived in office determined to end the wars in Iraq and Afghanistan. But he encountered two formidable opponents in Petraeus, the head of the U.S. Central Command, which coordinates military activities across the Middle East and Central Asia, and Lieutenant General Stanley McChrystal, who the new president had named the top commander in Afghanistan.

McChrystal’s effort began with a 66-page classified review of the war effort that reached the opposite conclusion to the one favored by Obama. In McChrystal’s assessment, he argued that he needed at least 40,000 more troops in order to succeed in Afghanistan. Petraeus, the respected architect of the surge who was being talked about as a future presidential candidate, stumped for additional troops in the media. Admiral Mike Mullen, the chairman of the Joint Chiefs, tacitly endorsed McChrystal’s plan in testimony before the Senate Armed Services Committee.

Then the classified McChrystal report was leaked to Washington Post reporter Bob Woodward. On September 21, 2009, the Post ran Woodward’s story on the front page: “McChrystal: More Forces or ‘Mission Failure.’” The impact of this leak is hard to overstate. It essentially forced Obama to take ownership of the Afghan War—a move he and his aides had resisted. If he stuck with his preferred option, he would be contradicting a respected general in the field who was predicting near-apocalyptic consequences unless additional troops were dispatched. At a speech at the International Institute for Strategic Studies in London, McChrystal was asked if a more modest approach could work. “A strategy that does not leave Afghanistan in a stable position is probably a shortsighted strategy,” he said. Obama was furious, according to Woodward’s book Obama’s Wars, telling his aides: “We’ve got to stop this.”

In the end, he couldn’t. In a December 1 speech at West Point, Obama ordered 30,000 new troops into Afghanistan. McChrystal could—and did—find another 10,000 soldiers from the ranks of the coalition forces, which meant he got his 40,000. The generals had won.

The military leaders that Trump would inherit are, by all indications, less forceful personalities than Powell, Petraeus or McChrystal, but it’s hard to imagine them responding well to this kind of behavior. The current Joint Chiefs chairman, Marine Corps General Joseph Dunford, is known as an especially skillful operator able to bridge the diplomatic and military worlds, with a low-key demeanor that helped to win over Obama. And in recent months, Dunford has found subtle ways to signal disapproval of Trump’s statements. When he was asked during a Senate Armed Services Committee hearing about Trump’s support for torture and attacking the families of suspected terrorists, Dunford didn’t mention Trump by name, but observed that such orders would have an “adverse effect” on “the morale of the force” and “aren’t legal for them to do anyway.” He also pushed back against Trump’s assertion that NATO was “very obsolete” by telling reporters, “In my mind, the relevance of NATO is not at all in question.” In an indication of how much Trump’s statements on national security have triggered intense debate within the military, Defense Secretary Ash Carter has made it clear he doesn’t want officials weighing in on campaign politics.

The nation’s intelligence agencies have also been gripped by similar anxieties about the prospect of a Trump presidency. CIA Director John Brennan recently told NBC News that he would refuse to execute orders from a future president to use waterboarding. John Rizzo, the former CIA general counsel who worked at the agency for 35 years, told me that the employees he still spoke with were “terrified” at the prospect of reopening the debates over the use of waterboarding and other harsh interrogation techniques. “Deep concern is a mild way of putting the prospect of their commander in chief basically ordering them to go down this road again given all the trauma,” he said. If Trump won and followed through on his proposals, Rizzo predicted an employee “exodus.” “At a minimum,” he went on, “people would refuse to participate in anything resembling the former interrogation program and insist on a transfer to another part of the agency where they wouldn’t be involved in these things.”

Meanwhile, with Trump still in a strong position to clinch the Republican nomination, some current members of the military bureaucracy are starting to game out exactly what they would do if he were elected. There are some who think Trump’s brash persona might actually be an asset. Wilkerson, the retired Army colonel, told me he’d recently spoken to an active-duty colonel who thought Trump might force through welcome changes to the military establishment. “If Trump comes in and shakes up these graybeard idiots,” Wilkerson said the colonel told him, “I’m all for it.”

But such sentiments seem very much the minority view. Earlier this year, Bryan McGrath, a defense consultant and former Navy officer of 21 years, watched in shock as Trump climbed to the top of the Republican field. He dusted off an anti-Trump manifesto he’d written the previous fall and sent it to Eliot Cohen, a former State Department adviser and professor at Johns Hopkins University. McGrath and Cohen polished up the letter, sent it to their contacts in the foreign policy community, and on March 2 published it on the website War on the Rocks with nearly 100 signatories. The letter charged that Trump would “make America less safe” and was “utterly unfitted to the office.”