It remains unclear whether the Trump administration will seek to take any additional detainees from the Syrian Democratic Forces as the situation in northern Syria continues to rapidly deteriorate after Mr. Trump’s decision to clear the way for Turkey to launch its operation into northern Syria.
The move is bringing to an abrupt crisis a long-simmering problem: About 50 countries have citizens in the Kurds’ prisons for ISIS fighters — and in the displaced persons camps where tens of thousands of ISIS women and children are held — and have been reluctant to repatriate them, instead leaving them in the Kurds’ hands indefinitely.
The male fighters the Kurds are holding include about 9,000 local Syrians and Iraqis, as well as 2,000 foreign fighters — including scores from Western Europe. Many European law enforcement officials fear that if they repatriate their extremist citizens, they would be unable to convict them or keep them locked up for a long time.
After Britain declined to bring Mr. Elsheikh and Mr. Kotey home for prosecution, instead stripping them of their citizenship, the United States government weighed various options for handling them itself before deciding to prosecute them in civilian court once it obtained all of the evidence it needed.
The Trump administration also weighed sending the two British men to the American military prison at Guantánamo Bay, Cuba, for a period of indefinite wartime detention without trial. Senator Lindsey Graham, Republican of South Carolina, who is a close ally of Mr. Trump’s but has criticized his Syria policy, has advocated that step.
But the military opposes getting more deeply involved in long-term detention operations, and there are steep legal obstacles to bringing the men to Cuba.
Among those challenges, transfer restrictions Congress imposed to block President Barack Obama from carrying out his plan to close the Guantánamo prison would make it illegal to transfer the men, once at the base, to domestic American soil for an eventual trial before a civilian court, and the military commissions system at Guantánamo is widely seen as too dysfunctional.