DOHUK, Iraq — The United States had encouraged its Kurdish allies to dismantle their defenses in northern Syria, saying it would make it easier to assure Turkey that the Kurds posed no threat. So in recent months, according to three American officials involved, the Kurds blew up tunnels and destroyed trenches, leaving themselves vulnerable as the United States promised that it would have their back.

Now, a week after President Trump’s decision to pull American support from them, the sense of betrayal among the Kurds, trusted allies now being forced to flee under assault from Turkey, is matched only by their outrage at who will move in: Turkish soldiers supported by Syrian fighters the United States had long rejected as extremists, criminals and thugs.

“These are the misfits of the conflict, the worst of the worst,” said Hassan Hassan, a Syrian-born scholar tracking the fighting. “They have been notorious for extortion, theft and banditry, more like thugs than rebels — essentially mercenaries.”

The sudden empowerment of Turkish-backed Syrian militias is yet another swift turnaround in Syria’s eight-year-old war, and it was unleashed by President Trump’s decision to pull United States troops out of the way for a Turkish military incursion.