WASHINGTON — First, President Trump was going to pull all 2,000 American troops out of Syria immediately. Then he was going to slow down the withdrawal. Then he was going to leave troops in neighboring Iraq.
Now, in the latest about-face, Mr. Trump has agreed to leave about 400 troops in Syria — 200 in a multinational force in the northeastern part of the country and another 200 at a small outpost in the southeast, where they will seek to counter Iran’s influence throughout the country.
His decision to commit what one senior administration official described on Friday as a “couple hundred troops” to the multinational force, operating south of the Turkish border, came after European allies refused to send troops if the United States would not.
Mr. Trump’s national security adviser, John R. Bolton, pressed the president to make the decision on Thursday, this official said, amid signs that the Pentagon’s negotiations to put together a stabilization and monitoring force were foundering on European resistance.
Whether Mr. Trump’s decision will persuade skeptical European leaders was not yet clear. The White House was still following up with British and French officials on Friday. But it underscored the chaos that followed the president’s abrupt announcement of a pullout in December.
Mr. Trump began walking back the timetable for a troop withdrawal almost immediately, under fierce pressure from not only European allies and Republicans in Congress, but also his own military and national security advisers, who were blindsided by his announcement.
On Thursday, after news of the president’s latest decision broke, the White House further deepened the confusion. The press secretary, Sarah Huckabee Sanders, said in a statement that “a small peacekeeping group of about 200 will remain in Syria for a period of time.”
In fact, according to the senior official, the troops will not be peacekeepers; they will be deployed indefinitely and they will not be the only American troops to remain in Syria. Another 200 will stay at the outpost in al-Tanf, in Syria’s southeast, near its border with Iraq and Jordan.
All told, the proposed multinational force could number between 800 and 1,500 troops, the official said, nearly all from NATO countries. It is intended to secure a safe zone in an area that is still vulnerable to a resurgence of the Islamic State and an incursion by troops from Turkey, which wants to make sure Kurdish fighters cannot launch terrorist attacks across its border.
In ordering the withdrawal in December, Mr. Trump declared that the United States had vanquished the Islamic State. “Our boys, our young women, our men — they’re all coming back, and they’re coming back now,” he said.
But Mr. Trump, the senior official said, now recognizes there are still supporters of the Islamic State in northeastern Syria and is willing to maintain a combat-ready American ground force there to prevent the extremist fighters from regrouping. The United States will also continue to provide air support to strike Islamic State targets.
The official declined to set out the rules of engagement for the troops, saying that was a matter for the Pentagon. But he made clear they would not operate under a United Nations peacekeeping mandate.
Mr. Trump discussed the latest developments on Thursday with President Recep Tayyip Erdogan of Turkey, with whom he also spoke in December before announcing the troop withdrawal.
The decision to leave a residual force has been pushed hard by Gen. Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff, who visited Turkey with Mr. Bolton soon after Mr. Trump announced the withdrawal and began laying the groundwork for a multinational force.
General Dunford has told colleagues in the administration that he is using up most of his “seven miracles” to devise a sustainable Syria policy, a lighthearted reference to the seven miracles of Jesus Christ that are recorded in the Bible’s Gospel of John.
Since Mr. Trump’s decision to withdraw troops, General Dunford and senior military officials have been looking at how to keep up the American military presence in Syria. In particular, the Pentagon wants to keep working with American-backed Kurdish and Arab fighters in Syria who have borne the bulk of the fight against the Islamic State.
The troops stationed at al-Tanf would continue to train local Syrian fighters and monitor Iranian-backed militias in the area. They could also relay necessary information for directing airstrikes to targets.
Pentagon officials had planned to present their proposal to Mr. Trump in the coming weeks; they said they were pleasantly surprised when he suddenly endorsed it on Thursday. Mr. Bolton, who has been accused of keeping a close hold on information, informed the Pentagon “30 seconds” after his conversation with the president, the senior administration official said.
Mr. Trump’s decision to leave Syria alienated allies, who warned that if the United States left the fight against the Islamic State, so, too, would they. Leaders of the Syrian Democratic Forces, who have fought alongside American troops, threatened to release Islamic State war prisoners if the withdrawal went ahead.
At a security conference in Munich last week, British and French officials were adamant with their American counterparts and in interviews that neither country would commit to ground troops in Syria unless the United States did so as well. They continued to balk, even after American military officials argued that London or Paris was more likely to be attacked by a resurgent Islamic State than New York or Washington.
As chaotic as the process has been, some analysts said the United States was now actually back to a workable military policy in Syria, with enough troops on the ground to preserve the status quo in the northeast, and to keep allies, particularly the Kurds, on board.
Still, the White House’s announcement on Thursday left the Pentagon scrambling yet again. Defense Department officials referred all questions about the announcement to the White House.
The Pentagon was not expecting the decision so quickly and had anticipated delivering a final pitch to the president in a few weeks. Officials described a surreal atmosphere at the Pentagon among military leaders who work on Syria policy and no longer know what to expect from one day to the next.