President Trump on Sunday dismissed scathing criticism over his move to pull U.S. forces from Syria, doubling down on the decision while naming Deputy Defense Secretary Patrick M. Shanahan as acting Pentagon chief.

Mr. Trump posted a message on Twitter saying Defense Secretary James Mattis, who resigned in protest last week over the Syria withdrawal, will leave at the end of the year and Mr. Shanahan, a former Boeing executive, will take over Jan. 1.

The announcement capped a whirlwind 72 hours in Washington, with Mr. Trump facing an intense bipartisan backlash after fulfilling one of his central campaign promises: to bring American troops home and end what he dubbed “never-ending wars” in the Middle East.

The decision roiled lawmakers and international allies and upended years of U.S. foreign policy in the region, but Mr. Trump argued that the Islamic State in Syria has been defeated and that he is facing unfair criticism for declaring victory.

“If anybody but your favorite President, Donald J. Trump, announced that, after decimating ISIS in Syria, we were going to bring our troops back home (happy & healthy), that person would be the most popular hero in America,” the president tweeted. “With me, hit hard instead by the Fake News Media. Crazy!”

After resigning last week, Mr. Mattis said he intended to stay on the job until the end of February to ensure a smooth transition.

But Mr. Trump, reportedly irked by the outgoing secretary’s harsh critique of his entire foreign policy approach and the narrative that Mr. Mattis was constraining the president’s impulses, pushed for an earlier exit.

“I am pleased to announce that our very talented Deputy Secretary of Defense, Patrick Shanahan, will assume the title of Acting Secretary of Defense starting January 1, 2019,” Mr. Trump tweeted. “Patrick has a long list of accomplishments while serving as Deputy, & previously Boeing. He will be great!”

Mr. Shanahan, who took over as deputy defense secretary in July 2017 after a 30-year career at Boeing, was rumored to be on Mr. Trump’s short list for the next Pentagon chief.

Sen. Tom Cotton, Arkansas Republican; former Sen. Jim Talent, Missouri Republican; and Air Force Secretary Heather Wilson also were believed to be in the running.

It’s unclear how long Mr. Shanahan will serve in an acting capacity or when the president will make a formal nomination. The position requires Senate approval.

The next defense secretary will be tasked with implementing Mr. Trump’s troop withdrawal from Syria and a drawdown of half of the 14,000 U.S. forces in Afghanistan.

The president has found little support in Congress for those moves — even though many liberal Democrats long have pushed for the U.S. to bring troops home from the Middle East.

A handful of Republicans say Mr. Trump is doing the right thing by resisting decades of orthodoxy in the party and in mainline military circles.

“The president promised he’d be different,” Sen. Rand Paul, Kentucky Republican, said on CNN’s “State of the Union.” “This is the mainstream foreign policy problem of our country. We think we always have to be involved. Maybe we shouldn’t have to choose a side and be involved with war.”

Before the president’s announcement, Mr. Mattis, National Security Adviser John R. Bolton and a host of other officials urged the president to keep the 2,200 U.S. forces in Syria.

Brett McGurk, the president’s special envoy to the global coalition against the Islamic State, also stressed recently that the U.S. must remain.

Mr. McGurk resigned over the weekend, and Mr. Trump and incoming White House Chief of Staff Mick Mulvaney downplayed his role by saying they had never heard of him.

“I have no idea who that person is,” Mr. Mulvaney said Sunday on ABC’s “This Week.”

On Twitter, Mr. Trump referred to Mr. McGurk as a “grandstander.”

The president also said he personally resurrected Mr. Mattis‘ career by giving the retired Marine Corps general a “second chance” after President Obama removed him as head of U.S. Central Command in 2013.

“When President Obama ingloriously fired Jim Mattis, I gave him a second chance. Some thought I shouldn’t, I thought I should,” Mr. Trump said. “Interesting relationship — but I also gave all of the resources that he never really had. Allies are very important — but not when they take advantage of U.S.”

In his resignation letter, Mr. Mattis stressed the importance of maintaining U.S. alliances with foreign partners, particularly in the fight against extremism.

With the Syria withdrawal, many lawmakers say, Mr. Trump is sending the wrong signal to allies and could set off a domino effect of negative consequences.

Specifically, lawmakers and military insiders fear that Iran and Russia will gain greater influence inside Syria. They also say Mr. Trump’s move puts Israel at greater danger of attack from Iranian-backed groups, such as Lebanon’s Hezbollah, operating inside Syria.

More urgent are fears that U.S.-backed Kurdish forces battling the Islamic State could, without American protection, be slaughtered by the Turkish military. Turkey has designated the Kurdish Workers’ Party (PKK) to be a terrorist organization.

Critics argue that the president is disregarding those concerns and believes he alone knows best.

“I think the president has felt that, ‘I got this now,’ ” retiring Sen. Bob Corker, Tennessee Republican, told CNN on Sunday.

Mr. Trump fired back at his political rival later in the day.

“Bob Corker was responsible for giving us the horrible Iran Nuclear Deal, which I ended, yet he badmouths me for wanting to bring our young people safely back home,” the president tweeted. “Bob wanted to run and asked for my endorsement. I said NO, and the game was over.”

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