So what’s the alternative? As of earlier this month, there wasn’t one. “There does have to be a Plan B of what we do next,” said Michael Mulroy, a deputy assistant secretary of defense for the Middle East, at a Washington event on Syria days before Trump’s call with Erdogan. “I can’t declare what that is here today because, quite frankly, we haven’t developed it entirely.”

Still, U.S. officials recognized that other militant groups in the region, after seeming defeats, had mutated and regrouped before—and prison breaks had played a large part.

“We should remember that at the beginning of ISIS, they got a lot of their fighting, a lot of their combat power, by breaking people out of prison in places like Mosul,” Joseph Votel, the former commander of U.S. forces in the Middle East, said at an event in Washington last week. “Hundreds, thousands of fighters instantly joined like that.”