WASHINGTON — Over the course of 24 extraordinary hours this week, 17 years of American military policy was thrown out the window as President Trump spurned his defense secretary’s plea to keep United States troops in Syria and began the long process of pulling out of Afghanistan.
On Friday morning, America’s 1.3 million active-duty service members woke up to a new reality: Their leader, Defense Secretary Jim Mattis, had resigned over the Syria withdrawal and Mr. Trump’s rejection of international alliances, and everything he and other military leaders had told them through three presidencies had suddenly been abandoned.
Terrorists must be challenged abroad before they end up here at home? No longer. Americans must defend their friends and stand up to authoritarians in places like Russia and Iran? A thing of the past.
As Mr. Trump overturned the post-9/11 national security consensus, the reverberations spanned the globe. In northeastern Syria, Special Operations troops who just the day before were fighting in the dirt around Hajin, the last Islamic State stronghold in the country, were now telling their Kurdish allies that they would be leaving them alone in the fight.
A continent and an ocean away at the Pentagon, the 26,000 men and women who work there digested Mr. Mattis’s resignation letter, in which he laid out, in stark terms, that he was leaving because the commander in chief did not believe in “the strength of our unique and comprehensive system of alliances and partnerships” that has preserved peace for decades.
And in Afghanistan, half of the 14,000 Marines and soldiers there were learning that they would soon begin leaving the war, which has claimed almost 2,300 American military lives since 2001.
“U.S. Quietly Builds Helipad on Roof of Embassy in Afghanistan” read the headline in Friday morning’s Duffel Blog, the satirical site that is read daily at the Pentagon, in a reference to the iconic Vietnam War photo of the evacuation of Americans from Saigon.
The notion of retrenchment rankled American service members who are used to the idea of a proactive United States military that promotes freedom and democracy and protects the weak around the world.
“Everyone has been saying that Trump didn’t want to visit the troops in war zones because he questions the mission there,” said Jon Soltz, an activist Iraq war veteran. “I think it’s the other way around. He’s scared to go to Iraq or Afghanistan, like he was scared to go to Vietnam, so he pretends to question the mission.”
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Mr. Trump promised to find a replacement for Mr. Mattis soon, and Defense Department officials on Friday suggested a list that includes Senator Lindsey Graham, Republican of South Carolina; Energy Secretary Rick Perry; Senator Tom Cotton, Republican of Arkansas; the Navy secretary, Richard V. Spencer; the deputy defense secretary, Patrick Shanahan; and Jack Keane, the retired four-star general who was vice chairman of the Joint Chiefs of Staff.
Whoever Mr. Trump selects will have to guide the Defense Department’s men and women through a new inward-looking way of conducting military policy, one that seeks to avoid messy international entanglements, particularly those in war zones.
At MacDill Air Force Base in Tampa, Fla., Gen. Joseph L. Votel, the head of United States Central Command, was on the phone last weekend with Pentagon leaders, trying to figure out how to talk Mr. Trump out of his decision to withdraw American troops from Syria.
His commander in chief was, essentially, ordering him to abandon the fighting force — the Syrian Kurds — that had been the most effective in working with the American military in the fight against the Islamic State. The move would also cede influence in Syria to Russia and Iran.
By Monday, aides said that General Votel appeared to have accepted that Mr. Trump was not going to budge. During a staff meeting, an officer asked General Votel if American troops were coming home, according to a Defense Department official with knowledge of the meeting. General Votel gave a grim smile and did not answer.
The next day — Tuesday — General Votel called a handful of officers into his office and told them that the president had decided to withdraw all troops from Syria. In Washington, Mr. Trump’s top advisers were still trying to talk the president out of the decision, but Central Command had its marching orders, first among them to inform the Kurds.
On Tuesday night in Tampa, General Votel called his Kurdish friend Mazlum Kobane. For years, General Votel has held weekly conversations with Mr. Kobane — the commander in chief of the Kurdish group, known as the Syrian Democratic Forces — a man the Americans had come to trust for always keeping his word.
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On Wednesday morning, even after receiving word that the Americans would be leaving them, Kurdish fighters continued to battle against the Islamic State in northern Syria, Defense Department officials said, gaining two acres of ground near Hajin.
The Trump administration’s order to start withdrawing roughly 7,000 troops from Afghanistan in the coming months is an abrupt shift in the war there, America’s longest.
Mr. Trump has made no secret of his distaste for the Afghanistan war, and he had to be talked into the Pentagon’s Afghanistan strategy last year, which called for a troop increase to 14,000. Like his predecessor, Barack Obama, Mr. Trump has been loath to continue pouring American blood and treasure into a war that by all accounts, except the Pentagon’s, is a stalemate.
Unlike his predecessor, Mr. Trump made the decision to begin his drawdown without notifying either the Afghan government or the United States’ NATO partners, who have also put their blood and treasure into the American war effort there.
“We have seen the reports,” Oana Lungescu, the NATO spokeswoman, said in an email. She added that “our engagement is important to ensure that Afghanistan never again becomes a safe haven for international terrorists who could threaten us at home.”
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Hundreds of American troops in Africa will be reassigned, and the number of Special Operations missions on the continent is set to fall sharply under an existing Trump administration strategy to shift the focus from terrorist threats to traditional “great power” challenges from nations like China and Russia.
Defense Department officials say they expect most of the troop cuts and mission reductions to come from Central and West Africa, where Special Operations missions have focused on training African militaries to combat the growing threat from extremist Islamist militant groups.
The decision to cut back on troops in Africa follows an ambush in Niger last fall that killed four American soldiers and an attack in southwestern Somalia that killed another in June.
Mr. Trump has told South Korean leaders that he wants them to double the amount of money they pay for American troops on the peninsula, a demand that the Seoul government is rejecting. Just as Mr. Trump has questioned why the United States and South Korea need to conduct joint military exercises, so too has he questioned why Seoul is not paying more to have American troops on hand.
Under an agreement that is set to expire at the end of the year, South Korea pays $830 million a year for the close to 30,000 American troops stationed on the peninsula. Defense Department officials say that Mr. Trump has asked for $1.6 billion per year.