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.”…

Within days of Mr. Trump’s assent to the Turkish-backed latest advance, human rights groups accused the militias of indiscriminate attacks on residential areas and killing civilians, including a prominent Kurdish politician.

“They are basically gangsters, but they are also racist toward Kurds and other minorities,” said Elizabeth Tsurkov, a fellow at the Foreign Policy Research Institute. “No human should be subjected to their rule.”