As a result, the international crisis that many of his opponents feared for so long has finally arrived, but it is one of Mr. Trump’s own making and one that pits him against his own party and his own government. The Turkish assault on America’s Kurdish allies that he effectively facilitated reflects his foreign policy in its rawest Trumpian form.

It is a foreign policy built primarily on reflex and increasingly resistant to outside advice. Unimpressed by the national security establishment and uninterested in the tedium of traditional policymaking, Mr. Trump often demonstrates more faith in what some overseas strongman tells him than the soft-boiled guidance of the bureaucrats, diplomats, intelligence analysts and military officers in the Situation Room.

“This may be the riskiest national security decision that he’s made to this point,” said Richard Fontaine, the chief executive of the Center for a New American Security and a former aide to President George W. Bush. “I don’t know that we’ve learned anything new about the president’s decision-making style, but it does reveal the very significant risks that that style carries.”

Having upended Middle East policy with the flick of a Twitter finger, however, Mr. Trump now is trying to find a way to reverse the eminently foreseeable consequences as Turkish forces bombard the same Kurdish fighters who helped the United States overpower the Islamic State, also known as ISIS.

Defense Secretary Mark T. Esper called his Turkish counterpart on Thursday to stress that Washington opposed the incursion into Syria, and Mr. Trump talked of brokering some form of peace between Turkey and the Kurds. “I hope we can mediate,” he told reporters even as he contemplated sanctions against Turkey.

But the administration signaled that it was still giving the Turks a degree of latitude. A State Department official, who briefed reporters under ground rules that he not be identified, said the administration would impose costs if Turkey went “beyond the lines,” which he defined as ethnic cleansing or indiscriminate artillery and airstrikes directed at civilians. That would suggest that Turkish forces that crossed the border into northern Syria on Wednesday could hit Kurdish fighters hard, but not slaughter them, without provoking American reaction. At the same time, the Kurds have been counterattacking, firing off shells at Turkish border villages as the danger of escalation spiraled.

Ever since the president abruptly announced his decision on Sunday night to remove about 50 to 100 American troops who served as a sort of trip wire along the border, effectively clearing the way for the Turkish incursion, the administration has scrambled to catch up. Senior officials across the government were caught off guard, and many shared the outrage expressed publicly by Republican lawmakers who called the move an abandonment of American allies.

From the highest levels of the Pentagon to the rank-and-file Special Forces in northern Syria, there was a deep sense of betrayal as troops were ordered to leave behind Kurdish allies whom they had gotten to know and trust. The troops along the border have all now been pulled back and relocated to bases deeper in Syria — posts still guarded by Syrian Kurdish fighters.

Three American officials said all counterterrorism operations by the Syrian Kurds had been suspended while they focused on fighting the Turks, several dozen missions a day. A scaled-back Kurdish jailer force is still securing Islamic State detainees at 20 locations around Syria, at least for now.

American diplomats described frustration at the president’s deference to President Recep Tayyip Erdogan of Turkey, with whom he talked shortly before announcing his decision on Sunday, and insisted that the new Syria policy was being directed by Mr. Trump — not the State Department.

“Trump clearly does not consider the second-order impact of his decisions,” said R. Nicholas Burns, a former under secretary of state under Mr. Bush. “This one move to appease Erdogan is now costing him and the U.S. something that is difficult to earn back quickly: our credibility as a reliable ally.”

The Turkish invasion also revealed a bitter new facet in a long-established divide between the Pentagon and the State Department in handling international conflicts. Senior military officials in the past week have privately — but adamantly — accused Secretary of State Mike Pompeo of being too passive in dealing with Mr. Erdogan and of failing to push back on the Turkish leader to protect the Kurdish fighters allied with American troops.

But senior State Department officials on Thursday described hours of telephone calls to foreign leaders and other meetings in which Mr. Pompeo has sought to ease the tensions in northern Syria — including efforts to help broker a cease-fire between Turkey and the Kurds. One senior State Department official described various diplomatic exchanges that have involved Mr. Erdogan.

The tension between the State and Defense Departments also reflects divergent views of the Turkish-Kurdish conflict. Amanda Sloat, a former American diplomat who focused on Turkey during the Obama administration, said the State Department had long been more sympathetic to Turkey, which it sees as a stabilizing force in the Middle East, while the military came to see the Kurds as “their brothers in arms” against the Islamic State.

Until now, Ms. Sloat said, “Their perspectives — including a single-minded focus on defeating the Islamic State irrespective of the longer-term consequences — usually won the day.”

With the Islamic State now crippled, though, Mr. Trump returned to his longstanding desire to get out of the region. His view that the United States has little business getting in the middle of fights like the Turkish-Kurdish conflict reflects years of national fatigue with overseas adventures and resonates with many Americans.

Having failed recently to seal a peace agreement with the Taliban in Afghanistan, Mr. Trump clearly sees Syria as one place where he could make progress toward withdrawing. “I am trying to end the ENDLESS WARS,” he wrote on Twitter on Thursday.

If his actions leave the Kurds to the mercies of the Turks, Mr. Trump seemed only modestly concerned. After all, as he put it on Wednesday, the Kurds “didn’t help us with Normandy.” Their assistance against the Islamic State was selfinterested, he said, not an act of support for the United States.

Indeed, Mr. Trump and his allies picked up the theme that it was really President Barack Obama’s fault for tying the United States so closely with the Kurds in the first place.

At the same time, the timing has its own domestic risks for Mr. Trump. His decision to abandon the Kurds sent his more hawkish Republican allies in Congress into paroxysms of anger at the same time the president needs a unified party to stave off removal from office in his impeachment inquiry.

But Mr. Trump has pushed back at his critics, saying it was well past time to move on from wars in the Middle East. For nearly three years, he has been restrained from going as far as he wanted to fulfilling that goal, and his Syria decision was an act of rebellion from the establishment that has resisted him.

“We won, we left the area,” he told reporters on Thursday. “I don’t think we want to go back in.”