Some analysts played down the threat as a bluff, or a cry for attention, saying that if the Islamic State prisoners were freed they could sooner be expected to turn on the Kurds than thank them.

But a report by the Syrian Observatory on Human Rights said the Syrian Democratic Forces leadership was discussing the prisoners’ release because many of their home countries had refused to take them back. The Observatory, a London-based group with a network of citizen monitors throughout Syria whose work is widely considered credible, said the militia was also concerned that it would need all of its fighters to defend against a possible Turkish military invasion.

That invasion became more likely with Mr. Trump’s announcement, which was seen as giving Turkey a green light to carry it out. The American withdrawal helps Turkey in two ways: it abandons the Kurds, whom Turkey sees as a threat, and removes American troops from northeastern Syria, which President Recep Tayyip Erdogan said he will invade to attack Kurdish positions there.

An invasion, however, would not be free of risk.

“The problem for them is that if they do go in, there is always the possibility that they run into a protracted guerrilla struggle,” said Steven A. Cook, a Middle East expert at the Council on Foreign Relations.



The greater likelihood, analysts said, was that the Kurds seek an agreement with Syria’s president, Mr. Assad, that grants them limited autonomy in eastern Syria in exchange for their loyalty. Such an agreement would put Turkey into greater conflict with both Mr. Assad and the Russians.

To Prime Minister Benjamin Netanyahu of Israel, Mr. Trump’s decision came as a deep embarrassment heading into an election year in which his national-security credentials, closeness with Mr. Trump and obsessive focus on stopping Iranian expansion were central to his argument for another term. Mr. Netanyahu said he learned of the withdrawal decision in talks with Mr. Trump on Monday.

“This caught the Israeli government by surprise,” said Ofer Zalzberg, an Israeli analyst at International Crisis Group. “I don’t know whether indeed Trump called Netanyahu two days ago or not, but that’s a very short time between two close allies. That’s barely a heads-up.”