The U.S. is bringing home citizens who joined the Islamic State to stand trial, but some Western European and other nations refuse to — and a new watchdog analysis says ISIS could benefit from their inaction.

This decision to let ISIS-joining citizens avoid justice leaves approximately 800 Europeans among the 2,000 foreign ISIS fighters from 50 countries in legal limbo in northeast Syrian detention camps run by the Syrian Democratic Forces, a U.S.-allied coalition of mostly Kurdish militias, where 10,000 ISIS fighters are believed to be held. The Pentagon’s inspector general report, released Tuesday, warned that the SDF does not have the capability to “indefinitely detain” thousands of ISIS fighters in what are described as “pop-up prisons.”

Only seven countries — the U.S., Bosnia and Herzegovina, Italy, Kazakhstan, Kosovo, Morocco, and North Macedonia — announced they will bring back fighters to face charges. Italy is the only Western European country to do so. France complained about death sentences handed down in Iraq against a dozen French ISIS members, and the U.K. is stripping citizenship from fighters. Outside of Europe, the rest are mainly from former Soviet republics, the Middle East and North Africa, and South and Southeast Asia.

President Trump has repeatedly urged Western European countries such as Britain, France, and Germany in particular to take back homegrown militants to "put them on trial" for their crimes.

Ambassadors Jim Jeffrey and Nathan Sales, who focus on ISIS and counterterrorism, urged U.S. allies last week to take responsibility for their homegrown fighters.

Sales said this crisis is a priority and that the U.S. is leading by example and wants other countries to repatriate and prosecute. Sales said the U.S. brought back five U.S. ISIS members (four men and one woman), with one convicted and four pending charges.

A sixth ISIS member was brought back and indicted in Dallas last week.

A State Department representative told the Washington Examiner two other adult female ISIS members and 13 children were also repatriated. The spokesperson said they are aware of a "very small number of detainees" who have claimed U.S. citizenship and that they are working to verify and handle the claims.

The U.S. also wants to extradite two British ISIS fighters, dubbed “the Beatles”, to the U.S. for prosecution for their alleged involvement in the murders of American journalists James Foley and Steven Sotloff, American aid worker Peter Kassig, and other hostages. The U.S. opposes repatriating Hoda Muthana, however, claiming the so-called “ISIS bride” is not a U.S. citizen.

George Washington University’s Program on Extremism says 191 individuals have been charged in the U.S. for ISIS-related offenses, with 40% of those cases involving traveling or attempting to travel abroad. A 2017 U.N. study estimated 40,000 fighters from 110 countries traveled to Iraq and Syria to join ISIS. A 2018 report by the International Centre for the Study of Radicalisation put that number at 41,490 fighters from 80 countries.

Jeffrey said of the 10,000 fighters "under lock and key," roughly 8,000 are Iraqi or Syrian. There are ongoing efforts "to move the Iraqis back to Iraq and for the Syrians to be placed on trial.”

But Jeffrey said “a bigger problem” is the sprawling al-Hawl refugee camp, which holds 70,000 ISIS family members and supporters. He said 60,000 are Iraqis and Syrians, but 10,000 are foreigners connected to the 2,000 foreign terrorist fighters in Syria.

“There are a variety of humanitarian issues that we’re working our way through in al-Hawl, but there’s also a problem of radicalization,” Jeffrey said. “In the long run, what we’re trying to do is to get people out and back into their communities.”

The inspector general report also warned that ISIS is “likely working to enlist new members from the camp’s large population” of internally displaced persons. United States Central Command strongly recommended moving the ISIS family members to Syrian “guarantors” or to Iraqi custody if the refugees are Syrian or Iraqi natives, and urged outside countries to take their own foreign citizens back, calling this “critical to reducing ISIS’s recruiting pool.”

Sales also said those detained by the SDF are being treated “humanely,” but these prison camps are ad hoc, temporary holding facilities.

“We’ve seen a number of attempted jailbreaks,” Sales said. “The risk that they could get out is not trivial.”

The Defense Department’s inspector general reported that “minimal security” at the camp “created conditions for ISIS’s ideology to spread uncontested.”

The State Department representative told the Washington Examiner that the U.S. is providing the bulk of the money to take care of thousands of detained fighters but that the effort is still “a tremendous challenge and burden” for the SDF. The representative said the Iraqi government is grappling with the strain of detaining so many fighters, too.

The fighters are “dangerous battle-hardened terrorists” who “left comfortable lives to go to the desert to fight for an idea,” Sales said, and countries have an obligation to ensure they don’t return to the battlefield.

“The reality is the way to be tough on foreign terrorist fighters is to prosecute them, but right now, they are not facing justice for the crimes they have committed,” Sales said.

Sales said no one should expect the U.S., SDF, or Iraq to offer justice. The best way for countries to show they’re serious about fighting terrorists is to “bring them home, put them in front of a court, have them tried, and then if they’re convicted, make sure they serve lengthy sentences,” he said.

“This is a problem that fundamentally is owned by the countries where their citizens were radicalized back at home,” Sales said.

Sales told the BBC's Newsnight that countries like Kosovo and Kazakhstan are doing a better job of prosecuting their fighters than wealthy Western democracies. He called it a “dereliction of responsibility” to “wash one’s hands of the problem.”