On Thursday he became the first defense secretary in American history to resign in protest over a presidential decision, according to presidential historian Michael Beschloss. And there’s no clear replacement in sight.

The news sent ripples of fear through Washington’s defense and foreign policy circles, among both Democrats and Republicans, and raised questions about whether a truly qualified candidate could be convinced to accept the task of running Trump’s Pentagon. Mattis plans to officially step down on Feb. 28.

“What serious person is going to accept that job, and come work for a president who changes his mind like people change their clothes?” said Robert Deitz, who held senior positions in the CIA and National Security Agency under former president George W. Bush. “Nobody comes to mind, mostly because they’d be unlikely to accept.”

Trump’s shock decision Wednesday to pull U.S. troops from Syria , which prompted Mattis’ abrupt resignation, is exactly the kind of handbrake U-turn in U.S. foreign policy likely to become even more frequent once Mattis’s influence fades, former senior national security officials told VICE News.

“I’m extremely concerned,” said Michael Carpenter, a former deputy assistant secretary of defense and foreign policy adviser to former Vice President Joe Biden. “His departure means we’ll have a president whose irrational impulses go even more unchecked.”

“Fucking kill” Assad

In 2017, an enraged Trump phoned Mattis and told him to “fucking kill” Syrian dictator Bashar Assad, following Assad’s reported use of chemical weapons, according to veteran D.C. journalist Bob Woodward’s book, "Fear."

“Let’s fucking kill him!” Trump said. “Let’s go in. Let’s kill the fucking lot of them.”

Mattis played along with Trump, then hung up.

“We’re not going to do any of that,” Mattis muttered to an aide, before drawing up plans for a more limited airstrike that Trump later agreed to. (Trump and Mattis have both denied this ever happened.)

Mattis was the last of three military generals installed at the top of Trump’s administration once widely seen as providing a check on Trump, alongside former National Security Adviser Gen. H.R. McMaster, and outgoing chief of staff Gen. John Kelly — dubbed by Trump as “my generals.”

“The message is that Trump is unable to listen to the sound voice of military experience.”

“We were led to believe at the beginning of the administration that Trump had this enormous respect for these retired generals, and that he’d defer to them on matters of security,” said Brett Bruen, U.S. Diplomat who served as director of global engagement in Obama’s White House. “But the message is that Trump is unable to listen to the sound voice of military experience.”

Mattis’s exit results from Trump’s “disturbing vision” of the world as a place where America’s allies are expendable, said Ryan Goodman, a former special counsel at the defense department.

“Secretary Mattis's letter of resignation is the canary in the coalmine,” said Goodman. “Mattis has told the nation that the president is no longer capable of making the types of reasonable decisions that keep our country safe.”

Trump’s decision to withdraw from Syria, in particular, throws two sets of American allies under the bus, former diplomats said: European nations who have fought in Syria, notably France and the U.K., and also local Kurdish forces, who have suffered heavy losses taking ground from ISIS fighters.

“We’re abandoning our European allies and the Kurds with no notice,” said Hady Amr, who was a top State Department official under former president Barack Obama. “They essentially found out about it through the internet. That’s not the respect with which we should be treating our allies.”

Trump’s wild swings appear likely to become even more rash as legal pressure increases from mounting criminal investigations into his presidency, former government officials said. The White House did not immediately respond to request for comment on Trump's foreign policy.

Reality show military

Several former top government officials worried openly that Mattis’s departure may enable Trump to play politics with the armed forces.

Trump has displayed a willingness to use the U.S. military like a prop in a reality show, former officials said, citing his decision to deploy thousands of troops to the U.S.-Mexico border to deter a caravan of unarmed migrants right before November’s midterm election, and his plan to roll heavy armor down the streets of Washington, D.C., in a military parade.

Multiple sources invoked the 1997 movie "Wag the Dog," a comedy about a president who concocts a phony war in Albania to distract the public from a sex scandal right before an election.

National security experts worried openly about how Trump will react, in Mattis’s absence, if special counsel Robert Mueller delivers damning findings about Trump’s ties to Russia or other crimes.

“The legal noose is tightening around his neck, and the absence of a key check on his foreign policy instincts means we’re in for very dangerous time,” said Deitz. “The potential for some 'Wag the Dog' scenario is much greater now than before we knew that Mattis was resigning.”

Some fear Trump will use the military to divert attention from the Mueller probe or his other legal problems.

“I think he instinctively knows that if he finds the right cause for military intervention, he could put the Mueller news cycle behind him and create new facts on the ground that could rally people behind him,” Carpenter said.

Trump himself once accused Obama of preparing to start a war with Iran to win re-election, a worrying sign that he sees foreign adventurism as a way to juice a president’s domestic ratings, former officials said.

“Donald Trump’s poor leadership continues to steer our country into chaos,” Rep. Eric Swalwell, a Democrat on the House intelligence committee, told VICE News. “Without General Mattis’s counsel, we are at risk of further alienating our allies and getting too close to the worst actors in the world.”