“All of the European countries, especially those with most of the foreign fighters, have desperately been looking for the past year for a way to deal with them without bringing them back,” said Rik Coolsaet, an expert on radicalization at the Egmont Institute, a Brussels-based research group. “But now, European nations are being forced to consider repatriation since Turkey is going to put people on the plane.”

The sudden problem for Europe is a long-tail consequence of President Trump’s precipitous decision last month to withdraw American forces from northern Syria, which cleared the way for Turkey to take control of territory as well as many of the Islamic State members who had been held there in Kurdish-run prisons or detention camps.

The issue is further complicated by the fact that nearly two-thirds of the Western European detainees, or about 700, are children, many of whom have lost one parent, if not both.

Now that more of the former fighters are in Turkish hands, Mr. Erdogan has not hesitated to use the threat of returning them as leverage over European countries who have been deeply critical of his incursion, and who have threatened sanctions against Turkey for unauthorized oil drilling in the eastern Mediterranean off Cyprus.

The fate of the former fighters and their families has become yet another point of contention between Turkey and Europe, which is already paying Mr. Erdogan’s government billions of dollars to stem the flow of asylum seekers from conflicts in Iraq, Syria and Afghanistan.