WASHINGTON – The resignation of Defense Secretary Jim Mattis over President Donald Trump’s abrupt decision to withdraw troops from conflicts in the Middle East and his denigration of allies has ruptured his bromance with military brass and mayscare off qualified successors, according to former senior officials and military experts.

Mattis, once hailed by Trump as one of “my generals,” follows a trio of former high-ranking military officers who were fired, pushed out or quit. Trump’s mercurial decisions were an affront to their experience and integrity – traits the president touted in his generals-turned-aides – said former Defense Secretary Chuck Hagel.

Mattis had no choice but to resign after Trump disregarded his advice to keep roughly 2,000 troops in Syria, said Hagel, a former Republican senator who served in President Barack Obama's administration. Finding a competent successor, he said, will be nearly impossible.

“Anybody with character and integrity would be hard-pressed to take that job,” Hagel told USA TODAY. “It never ends well for anybody.”

The shake-up ended the era of Trump's reliance on generals as top aides in his administration. Trump’s early embrace of high-ranking military men was so strong some critics worried military leaders were exerting too much influence over the inner-workings of civilian government. No such worries now.

More:Defense Secretary Jim Mattis leaving his job after clash with Trump over Syria

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What became known as the “my generals” club included four-star Marine generals Mattis and JohnKelly, who served as the Homeland Security secretary and later chief of staff. Trump also named two, three-star Army officers as national security adviser: Michael Flynn and, later, H.R. McMaster when Flynn was fired for lying to Vice President Mike Pence.

All four men have either departed the administration or will do so in a matter of weeks.

Speaking to members of Congress hours after his inauguration, Trump made his feelings about former military leadersclear by usinga phrase he would repeat over and over during his first year in office: “My generals.”

“I see my generals – generals are going to keep us so safe,” the president said, pointing specifically to Mattis and Kelly. “These are central casting – if I’m doing a movie, I pick you, general.”

The former generals – except Flynn – won praise and gratitude from members of both political parties who viewed them asa check on the president's nationalist impulses. Because of that, the club drew skepticismfrom Trump supporters who wanted himto speed up his agenda.

“Those generals formed what might be called an integrity block. They were collectively, with the single exception of Flynn, people of integrity who would do their best for the country and would try to get things done,” said Thomas Kolditz, a retired brigadier general and the director of the Doerr Institute for New Leaders at Rice University.

“What we’re seeing, I think, is an across-the-board collapse in persons of integrity supporting the president,” he said.

But in a clear sign their influence had waned, Trump announced this week that he planned to pull U.S. special operations troops who had been training local security forces in Syria fighting ISIS terrorists – a withdrawal that had beenopposed by Mattis and others at the Pentagon. The president is also considering a major drawdown of U.S. military in Afghanistan over the objection of some in his administration.

Ordering the precipitous retreats from Syria and Afghanistan and backing off a commitment to avoid a partial shutdown of government show Trump's ignorance in running the government, Hagel said.

“Day after day we have more of these. I don’t think the president has a foreign policy or a strategy about anything because he doesn’t understand government,” Hagel said.

The abrupt reshuffling of military strategy prompted some Trump critics to speculate about what changes the president may make next in the absence of Mattis and Kelly.

Trump repeatedly questioned the U.S. military’s commitment on the Korean Peninsula and canceled military training exercises there after his summit last summer with North Korean leader Kim Jong Un.

And he has criticized post-World War II alliances like NATO and the United Nations so often that some observers suggest U.S. influence in those organizations has declined. Both Mattis and Kelly had defended the need for those entities.

David Lapan, a former aide to Kelly and a retired Marine colonel, said he is concerned the fallout from the departures will send a dangerous signal abroad.

“I think we should be very concerned that Russia will see this as a weakness and will take further steps against Ukraine and potentially against other countries in that region,” said David Lapan, now with the Bipartisan Policy Center.

The decisions to draw down from Afghanistan and Syria, he said, "are going to embolden ISIS and other terrorist organizations."

Hagel and others said they are concerned that Trump's decisions will also complicate his efforts to find credible replacements. Several potential candidates took themselves out of the running to serve as White House chief of staff.

Trump has described Kelly's successor, Mick Mulvaney, as an "acting" chief.

Sen. Tom Cotton, R-Ark., Deputy Defense Secretary Patrick Shanahan and former Sen. Jim Talent of Missouri are among the candidates expected to be considered to replace Mattis. Trump said Thursday that a new Defense secretary would be named "shortly."

The top Democrat on the Senate Armed Services Committee, Jack Reed of Rhode Island, said he doubts Trump will find a replacement with Mattis' qualifications.

"It’s going to be very difficult," Reed told USA TODAY. "Mattis seems to be a casualty in the president's constant need to satisfy his ego."

Trump does retain one general he hand-picked for a top post, Marine Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff. As such, Dunford is the principal military adviser to the president. It was Obama who first chose Dunford to serve a two-year term, and Trump gave him the customary second term early in his presidency.

Dunford and Mattis, close colleagues, rose through the Marine Corps ranks, fought with allies in Iraq and Afghanistan, and share the same reverence for American alliances. It’s likely they have voiced similar concerns to Trump about the need to stay engaged in Syria and Afghanistan with NATO and other partner nations.

Dunford’s term is set to expire next fall, but in an unusual move, Trump has alreadyannounced his successor: Army Gen. Mark Milley.