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 Erdoğan. “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.”

The ISIS leader Abu Bakr al-Baghdadi did time in a U.S.-run prison camp in Iraq. “It was within the confines of these overcrowded camps that the next iteration of terror germinated among detainees,” a brief from the Soufan Group, a private intelligence firm, noted in April. What would become the Islamic State’s leadership recognized the potential of these prisoners, and organized a campaign to free them.

The dynamic could well repeat itself in the cauldron of northern Syria today. The United States has taken custody of two of the most famous ISIS prisoners—the British-born men Alexanda Kotey and El Shafee Elsheikh, who are accused of being part of a cell that imprisoned, tortured, and beheaded Americans. The New York Times has reported, citing American officials, that the Kurds refused to transfer other prisoners to U.S. custody. On Sunday, Esper announced that the Turkish offensive was broader than the U.S. originally anticipated, and that 1,000 U.S. troops would be pulling out of northern Syria altogether, leaving them with even less insight into the network of Kurdish-run prisons in the area.

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The Kurdish People’s Protection Units (YPG) have reportedly reduced but not abandoned their presence at the prisons. “The threat is more that the expanding war between Turkey and the YPG will engulf northern Syria and indirectly result in security gaps that allow ISIS to escape, either opportunistically or through coordinated attacks against limited security forces at the detention facilities,” Jennifer Cafarella, the research director at the Institute for the Study of War who has tracked the Syria conflict, wrote in an email.

The risks have spiked, but the policy is the same: Countries just need to take their citizens back. Most were unwilling before, and now they may be unable. As security deteriorates in northern Syria, there’s no obvious way to get prisoners out and back home even if their governments wanted them. Trump has suggested that Turkey can take over, but Turkey has shown little inclination so far. Trump officials meanwhile reiterate that sending the fighters home is the best solution. “We have tried very hard, over a period of months, to get the home countries for these detainees to take responsibility for them,” said a senior official on the phone call with reporters this week. “We think that is the logical resolution for this situation.”