President Trump’s decision to pull American forces from key posts in Syria set U.S. counterterrorism officials on edge Monday amid concern that the move will lead to the headlong release of thousands of hardened Islamic State fighters from makeshift prison camps there.

Amid the geopolitical concerns and the warnings about U.S. strategic goals and reliability, the massive al-Hol detention camp run by the now-isolated U.S. Syrian Kurdish allies, holding thousands of hardened Islamic State fighters, may pose the most immediate security challenge.

Mr. Trump for months has fumed that Western European nations in particular have refused to take custody of hundreds of the captured fighters who are believed to hold European Union citizenship and are detained in facilities run by the network of Kurdish militias known as the Syrian Democratic Forces, or SDF.

The SDF’s ability — and willingness — to administer the prison is in serious doubt after Mr. Trump’s withdrawal decision. SDF leaders warned that they will have to transfer the guards and security forces from the prisons to the front lines if a clash with Turkey ignites.

While the SDF has U.S. funding for the prisons, as well as behind-the-scenes support from U.S. Special Forces, a White House statement Sunday night suggested that Mr. Trump is eager to go in a different direction.

“Turkey will now be responsible for all ISIS fighters in the area captured over the past two years,” said the statement, using the acronym for the Islamic State.

Although the bulk of the ISIS captives hail from Syria, Iraq and other Middle Eastern nations, Mr. Trump — at a White House briefing that was supposed to focus on a Japanese trade deal — again focused on the refusal of European countries to take home their nationals who traveled to the region in support of the ISIS “caliphate.”

Senior U.S. counterterrorism officials were trying to contain the fallout from Mr. Trump’s surprise move. They argued that it would be a “misinterpretation” to say the White House invited Turkey to sweep in and militarily take over the Kurdish-run prison camps.

Mr. Trump has not ordered a withdrawal of U.S. personnel from the area of northeastern Syria where the prison camps are located, the officials said on background.

However, concerns were soaring Monday night that Turkish forces may seize on confusion about the U.S. position to invade the area and target Kurdish militias that Ankara insists are linked to a long-running militant Kurdish separatist movement inside Turkey.

“If Turkish forces actually move in, the SDF are going to have to make a major decision on whether to go fight the Turks and protect the border around their territory or stay where they are and keep up security around the prisons,” one senior official told The Washington Times. “It’s an extremely delicate situation, and it’s something we’re extremely concerned about.”

While Turkey pondered its options, Mr. Trump appeared to contradict his own White House statement by threatening on Twitter to “totally destroy and obliterate” the Turkish economy if Ankara does launch a war to drive Kurdish militias from their posts on the Syrian side of the border.

National security analysts say the administration badly needs to clarify its plan for the Islamic State prison camps.

“U.S. officials need to have a clear-cut contingency plan lest the security situation devolve to a point these guys get released,” said Bill Roggio, a counterterrorism analyst with the Foundation for Defense of Democracies. “If that happens, it would represent a major and really regrettable blow to the gains made against Islamic State over the past four years.

“The reality is that the U.S. shouldn’t disengage until the prisoner issue is dealt with, but the other reality is that it appears the prisoner issue is not being dealt with,” said Mr. Roggio, an editor of the foundation’s Long War Journal.

Building frustration

Mr. Trump’s push for U.S. allies to take custody of foreign-born Islamic State fighters last made headlines in August, when his frustration on the matter burst into the open on the sidelines of the Group of Seven summit of leading industrial nations in France.

The president told reporters that the U.S. won’t foot the hefty bill of holding the jihadis at the U.S. detainee center at Guantanamo Bay, Cuba. He said the U.S. might be forced to release detainees back into their homelands if countries such as France and Germany continued to balk at taking them.

Administration officials have suggested that the number of captured fighters with European Union citizenship is higher than 800, although U.S. counterterrorism officials have declined to comment on the figure.

The officials say some 2,000 fighters held by the SDF in northeast Syria are a mix of some 50 nationalities, including from North African nations, Gulf Arab states, Russia and Western Europe.

Some countries, including Russia, Sudan and Malaysia, have agreed to take back their nationals, but others have ignored the U.S. calls. Western European nations in particular are wary of political blowback that could come with taking on the cases of prisoners who hold dual citizenship in North African and European Union nations.

Laws in most European countries would hinder convictions without explicit evidence of defendants’ roles as Islamic terrorists. In the absence of legal reforms, the risks are high that fighters returned to Europe could be quickly released.

The situation is also sticky for Washington. A CBS News reporter gained access to a prison run by the SDF last month and met a number of inmates who claimed to hold U.S. citizenship. While thousands of EU citizens are reported to have traveled to Syria to fight with ISIS during its 2014-2016 heyday, a much smaller number are believed to have traveled from the United States.

Some legal analysts blamed Mr. Trump for adding fuel to an already volatile situation.

“Many of us have long warned that this detention status quo was not sustainable over the long haul because of the risk that SDF forces at some point might not be able to continue to hold the requisite territory or because they might for some reason cease to be willing to administer this detention system themselves,” said Robert Chesney, a national security lawyer at the University of Texas School of Law.

“Now it seems that … triggering conditions for a collapse of the SDF detention system are looming before us, thanks to President Trump’s precipitous decision to greenlight a Turkish invasion of northern Syria,” Mr. Chesney wrote Monday for Lawfare, a national security blog published with the Brookings Institution.

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