The president’s remarks capped an extraordinary month in which the United States reversed its yearslong policy in northern Syria and abandoned Kurdish fighters who joined American forces to defeat the Islamic State.

Turkey, a North Atlantic Treaty Organization ally, views the Kurdish fighters as terrorists. In an Oct. 6 telephone call, President Recep Tayyip Erdogan informed Mr. Trump that he would invade the Kurdish-held region in northeast Syria to combat them; the American leader responded by ordering United States troops out of the way.

But many Democrats and some prominent Republicans have called the American retreat a historic foreign policy debacle that undermines Washington’s credibility with allies, empowers American rivals and gives the Islamic State a chance to regroup.

“President Trump seems determined to keep handing political and military victories to Russia and Syria, kowtowing to Turkey and opening the door for further Iranian expansion in the region,” said Senator Robert Menendez of New Jersey, the top Democrat on the Foreign Relations Committee.

In the weeks since his call with Mr. Erdogan, Mr. Trump has repeatedly returned to his 2016 campaign themes of concluding “endless wars” and avoiding “regime change.”

But Senator Lindsey Graham, Republican of South Carolina, said the Trump administration had, in fact, pushed Turkey into the initial pause in violence. And he warned against allowing Russian, Turkish or Syrian government forces to police the northern Syrian border, doubting their ability to ward off the Islamic State.

“While I agree that America is not the policeman of the world, I firmly believe the American military is the most capable to protect America, and should be used wisely to do so,” Mr. Graham said in a statement. He said American air forces should patrol the skies above the safe zone.