WASHINGTON — President Trump vowed on Monday to pull back from military involvement in the Middle East and leave it to others “to figure the situation out,” even as one of his strongest supporters condemned him for abandoning allies and emboldening regional enemies.
In a series of Twitter messages, the president defended his decision to clear the way for a Turkish military operation that would sweep away America’s Kurdish allies near the Syrian border, arguing that the internecine conflict among forces in the region was not a top priority for a war-weary United States.
“I held off this fight for almost 3 years, but it is time for us to get out of these ridiculous Endless Wars, many of them tribal, and bring our soldiers home,” Mr. Trump wrote. “WE WILL FIGHT WHERE IT IS TO OUR BENEFIT, AND ONLY FIGHT TO WIN. Turkey, Europe, Syria, Iran, Iraq, Russia and the Kurds will now have to figure the situation out.”
The president’s abrupt decision overrode the objections of the Pentagon and State Department, which sought to maintain a small American troop presence in northeastern Syria, and caught even some of Mr. Trump’s top allies off guard. One of the top Republican hawks in Congress quickly castigated the president and promised to try to sanction Turkey if it followed through with its plans.
“If I didn’t see Donald Trump’s name on the tweet, I would have thought it was Obama’s rationale for getting out of Iraq,” Senator Lindsey Graham, Republican of South Carolina and a top backer of the president’s, said on Fox News.
As with President Barack Obama’s decision to pull out American troops from Iraq in 2011, Mr. Graham said, this would create a vacuum for remnants of the Islamic State, President Bashar al-Assad of Syria and others to surge forward again.
“This is a big win for Iran and Assad, a big win for ISIS,” Mr. Graham said. “I will do everything I can to sanction Turkey if they step one foot in northeastern Syria. That will sever my relationship with Turkey. I think most of the Congress feels that way.”
Mr. Graham said he would also introduce a nonbinding resolution asking Mr. Trump to reconsider his move, which he called “shortsighted and irresponsible.” The president’s assertion that the Islamic State has been defeated is “the biggest lie being told by this administration,” Mr. Graham added.
The announcement set off a swift and bipartisan backlash from other lawmakers as well, with some of the president’s closest allies on Capitol Hill taking to Twitter to denounce the decision, all while carefully avoiding the president’s name.
Representative Liz Cheney of Wyoming, a member of the House Republican leadership, called withdrawing United States forces from northern Syria “a catastrophic mistake.” Senator Marco Rubio, Republican of Florida, said it would be “a grave mistake that will have implications far beyond Syria.”
The Senate, led by Senator Mitch McConnell of Kentucky, the majority leader, previously relayed their displeasure with Mr. Trump’s isolationist instinct in Syria in January, when it voted overwhelmingly to rebuke Mr. Trump over his planned withdrawal of military forces from Syria and Afghanistan.
Mr. Trump came to office promising to get out of overseas wars, contending that the country’s military involvement in Afghanistan, Iraq and elsewhere since the attacks of Sept. 11, 2001, has been largely a waste of lives and money with little to show for it.
A similarly sudden decision last winter to pull American troops out of Syria prompted Defense Secretary Jim Mattis to resign and Brett McGurk, the special presidential envoy to the coalition fighting the Islamic State, accelerated his own planned departure in protest.
Mr. Trump later walked back his decision to some extent, but has been frustrated not to be doing more to extricate the United States from entanglements in the region.
His latest decision came after a telephone call with President Recep Tayyip Erdogan of Turkey. American officials indicated that the 100 to 150 United States military personnel deployed to northeastern Syria would be pulled back in advance of any Turkish operation but that they would not be completely withdrawn from Syria.
The Kurdish forces in the area, part of the Syrian Democratic Forces, or S.D.F., have been the most reliable American ally in the region for years, a critical element in recapturing territory once controlled by the Islamic State. But Turkey has long considered the Kurdish fighters to be terrorists and has lobbied the United States to abandon support for them.
“The United States was supposed to be in Syria for 30 days, that was many years ago,” Mr. Trump wrote on Monday. “We stayed and got deeper and deeper into battle with no aim in sight.” Now, he said, it is time to leave.
He offered little sympathy for the fate of America’s Kurdish allies: “The Kurds fought with us,” he wrote, “but were paid massive amounts of money and equipment to do so.”
Mr. Trump has been particularly irritated that the United States continues to pay to detain thousands of Islamic State fighters. For months, he has tried to pressure European states and others to take those fighters who originated from there, only to run into strong resistance.
“Europe did not want them back, they said you keep them USA!” Mr. Trump wrote. “I said ‘NO, we did you a great favor and now you want us to hold them in U.S. prisons at tremendous cost. They are yours for trials.’ They again said ‘NO,’ thinking, as usual, that the U.S. is always the ‘sucker,’ on NATO, on Trade, on everything.”
But if Turkey moves against the Kurds, the S.D.F. could abandon camps to fight the Turks, potentially allowing some 10,000 Islamic State fighters, including 2,000 foreigners, to escape. United States military officers were trying to reassure the S.D.F. in hopes of avoiding such a scenario.