With a bizarre threat to “release” terrorists into France and Germany, President Trump is pressing America’s European allies to bring home and put on trial their citizens captured while fighting for ISIS in Iraq and Syria.

Publicly elevating an issue that has bedeviled senior officials at the Pentagon and the State Department for months, Trump said he would order the transport of some 2,000 captured fighters back to their home countries if “Europe doesn’t take them”—something that would almost certainly be legally impossible.

“We're holding thousands of ISIS fighters right now. And Europe has to take them. And if Europe doesn't take them, I'll have no choice but to release them into the countries from which they came. Which is Germany and France and other places,” the president told reporters on Wednesday.

Later, in a speech to a veterans group in Kentucky, Trump said that European allies “say to us, ‘Why don’t you hold them in Guantanamo Bay for 50 years and spend billions and billions of dollars holding them’.”

If true, the suggestion would be a remarkable reversal for European leaders, who have spent close to two decades criticizing the United States for human rights abuses at the naval detention facility.

“And I’m saying, ‘No, you gotta take ‘em’,” Trump said.

In the past, senior Trump administration officials have emphasized that the United States is not holding the captured fighters. The U.S.-backed Syrian Democratic Forces, or SDF, is holding around 2,000 suspected foreign fighters in northeastern Syria, with American security support.

These foreigners are among 9,000 ISIS fighters currently held by the SDF. Just under a thousand are believed to be European. A small but unknown number are believed to be from the United States.

The Kurdish group is also holding around 70,000 ISIS wives and children, around 10,000 of whom are believed to be from places other than Syria and Iraq.

U.S. officials have been urging allies—largely unsuccessfully—to take back and try their own citizens. But European governments, concerned that the kind of battlefield evidence they have about the fighters won’t prove admissible in domestic courts, have so far resisted calls to repatriate either the men or their wives and children. The situation is straining the SDF’s capacity, and the U.S. has already been forced to call in surveillance support to help put down a jailbreak from a detention facility in Derik, in northeastern Syria.

Some countries—notably, France—have allowed their citizens to be tried in Iraq, where the courts provide minimal access to lawyers and typically mete out a hanging sentence on scanty evidence. (France does not have a death penalty.)

The United States has taken back and charged a handful of its own fighters, most recently a 23-year-old man born in Dallas now charged with providing material support to ISIS.

But the group of fighters pose significant challenges. In 2017, the United States held a U.S.-Saudi citizen for over a year in military detention because officials were certain in the intelligence that indicated he was an ISIS fighter, but believed that it wouldn’t be admissible in an Article III court. Charging him in federal court, they feared, would lead to his release. Civil rights advocates argued that to hold him indefinitely without charge violated his constitutional rights as a citizen.

Still, it is widely agreed that the SDF cannot hold the fighters indefinitely. In addition to the strain on its security resources, the group is not an internationally recognized sovereign government. It’s not clear that the group is even legally able to carry out law-of-war detention.