“Our boys, our young women, our men, they’re all coming back and they’re coming back now. We won,” Trump said in a video message on Twitter.

The step is the latest twist in a years-long effort by two administrations to defeat the militants in Syria and end the country’s long civil war. The conflict has made Syria a cauldron for terrorist threats and set the stage for a proxy battle that has included forces backed by the United States, Turkey, Iran and Russia. Since 2015, Moscow’s military support has been crucial in helping Assad turn the war in his favor.

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The decision also delivers on the president’s repeated threat this year to pull out troops. Since before taking office, Trump has promised to conclude the campaign against the Islamic State, also known as ISIS, and questioned the value of costly and dangerous military missions overseas.

Administration officials characterized Trump’s order as a response to the fact the Islamic State, which once ruled a vast swath across Iraq and Syria, had all but been defeated.

“We have started the process of returning U.S. troops home from Syria as we transition to the next phase of the campaign,” Pentagon spokeswoman Dana White said in a statement. “We will continue working with our partners and allies to defeat ISIS wherever it operates.”

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U.S. troops, working alongside Kurdish paramilitary forces, have struggled, however, to eradicate ­remaining pockets of militants.

Behind the scenes, the move generated confusion as military officials raced to outline plans for a rapid departure of the entire U.S. force of more than 2,000 troops. U.S. forces have been working in small numbers with local forces since 2015.

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A host of officials, including Defense Secretary Jim Mattis and Secretary of State Mike Pompeo, have dismissed a precipitous departure as premature and risky as militants are embracing insurgent tactics and seeking to regain strength in areas they once controlled.

“Getting rid of the caliphate doesn’t mean you then blindly say, ‘Okay, we got rid of it,’ march out, and then wonder why the caliphate comes back,” Mattis told reporters in September.

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Current and former White House officials, speaking on the condition of anonymity to discuss internal deliberations, described the announcement as a “knee-jerk presidential reaction” to Trump’s call last week with Turkish President Recep Tayyip Erdogan. The Turkish leader has threatened to launch a military offensive against Kurdish forces in Syria in coming days.

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Those forces are the chief U.S. partners on the ground in Syria, and U.S. troops remain positioned in areas that would be affected by a Turkish assault.

The officials said Trump had startled his own aides earlier this year by saying U.S. forces would depart “very soon.” Since then, Mattis, Chief of Staff John F. Kelly and others had repeatedly talked Trump out of ordering a swift exit, they said.

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“It was terrifying to us at one point when we’re trying to have thoughtful planning sessions about what to do in Syria, and he just said, ‘We’re getting out of there,’ ” one former official said. “He sees Syria as a totally useless desert.”

“People have bought time, bought time for months,” the official said. But the president complained that his wishes were being slow-rolled and expressed skepticism about Pentagon arguments that the United States had a responsibility to keep the region safe.

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Officials said national security adviser John Bolton, who recently outlined a policy to remain in Syria to counter Iranian influence, was among those who opposed the move.

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“I haven’t spoken to a person in the building who is happy about this,” a second official said.

The announcement came as a surprise to European and Arab allies, including some whose forces have fought alongside the United States in Syria. The Wall Street Journal and Reuters first reported the plans on Wednesday.

The opposition from Trump’s senior advisers suggested that the withdrawal could possibly be slowed or abandoned.

But a senior U.S. official downplayed the significance of the conflicting statements from Trump and his chief aides.

“The president’s statements on this topic have been 100 percent consistent from the campaign through his announcement today,” the official said told reporters. “And it was the president’s decision to make, and he made it.”

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Trump, whose video message appeared to be aimed directly at supporters to whom he promised a withdrawal from Syria, tied his decision to the loss of U.S. troops there. A small number of American service members have died in Syria, far fewer than in some in other recent wars.

The president seemed to suggest that slain troops would support his decision. “That’s the way we want it, and that’s the way they want it,” he said, pointing toward the sky.

U.S. forces have been positioned mostly in Syria’s north-central and northeast areas, which are now largely under the control of Syrian Kurdish partner forces. American troops also have a smaller ground presence in southern Syria along the border with Jordan.

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The withdrawal plans leave major questions for an alliance of U.S.-backed Kurdish and Arab militias, known as the Syrian Democratic Forces, or SDF, which have provided key aid for U.S. forces in the fight against the Islamic State.

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But the SDF is strongly opposed by Turkey, which sees any powerful Kurdish militia as a potential threat and further inspiration for Kurdish separatists inside Turkey.

A senior Kurdish official said the alliance would hold an “emergency meeting” to plan next steps.

Officials in Russia, which has repeatedly been condemned by the United States for bombings of civilian areas and support for Assad, expressed cautious satisfaction with the decision. Moscow has long described the U.S. mission in Syria as illegal because it wasn’t approved by the Assad government.

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“A very important consequence that might flow from this decision is that it could really, truly clear the way for a political settlement,” Russian Foreign Ministry spokeswoman Maria Zakharova said, according to Russia’s Tass state news agency.

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The dual message from Washington created skepticism in foreign capitals about whether the United States would actually follow through. Russian President Vladimir Putin has previously announced Russian withdrawals from Syria, but the Russian military remains active there.

As recently as last week, Brett McGurk, the presidential envoy for the global anti-Islamic State coalition, told reporters that “it’s fair to say Americans will remain on the ground after the physical defeat of the caliphate, until we have the pieces in place to ensure that that defeat is enduring.”

Any other policy, he said, would be “reckless. . . . I think anyone who has looked at a conflict like this would agree with that.”

Gen. Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff, suggested earlier this month that the military would be in Syria for an extended mission, saying the Pentagon had a long way to go in standing up local security forces capable of preventing the Islamic State from returning.

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Lawmakers from both parties immediately criticized the policy shift. Sen. Lindsey O. Graham (R-S.C.) said in an interview that he was “completely blindsided” by the announcement.

“With all due respect to the president, ISIS has not been defeated in Syria or Iraq and certainly not Afghanistan, where I just returned,” he said. “ISIS has been dealt a severe blow but are not defeated. If there has been a decision to withdraw our forces in Syria, the likelihood of their return goes up dramatically.”

Sen. Robert Menendez (D-N.J.) and Sen. Jack Reed (D-R.I.), the ranking members of the Senate Foreign Relations Committee and Senate Armed Services Committee, respectively, warned in a statement that the decision would benefit Russia and Iran, Assad’s other major patron.

“It is alarming to see President Trump running headlong into a hasty and poorly thought-out decision that could further abdicate U.S. leadership,” they said.

A small group of lawmakers applauded the move. Sen. Rand Paul (R-Ky.), who shares the president’s distaste for foreign wars, said on Twitter that he was “happy to see a president who can declare victory and bring our troops out of a war. It’s been a long time since that has happened.”