When the caliphate of the Islamic State collapsed, its fighters scattered.

Many ISIS terrorists ended up in camps being run by Syrian Kurds in northern Syria. Among the number are about 2,000 Europeans, and what exactly to do about that is the biggest source of disagreement that exists between the U.S. and Europe on counterterrorism policy today.

To the frustration of the U.S., the Europeans have shown no desire to take back their citizens.

President Donald Trump lambasted the Europeans for that and warned that unless they act fast, the U.S. will be forced to simply set free the Europeans being detained. (U.S.-backed Kurds have day-to-day control of the camps.)

Ambassador Nathan Sales, the State Department’s coordinator for counterterrorism, described the European approach as a “dereliction of responsibility.”

This sounds like it should be a simple problem to fix. Those traveling to Syria as soon as ISIS declared its caliphate were in all likelihood going there to live under the Sharia-law state controlled by the terrorists.

They are now captured, so why don’t the Europeans just take back their citizens and throw them in prison?

However, strongly suspecting something does not necessarily translate to convictions in European courts, where the burden of proof is understandably high.

We may think the stories of men like Shabazz Suleman—who gave an interview to Sky News while in Kurdish captivity, saying he just spent his time in Raqqa, Syria, “playing PlayStation or going around on bike rides”—are ridiculous. And they are, but proving it in court is no easy task.

Furthermore, think of the technical difficulties of prosecuting those detained in Syrian camps. Some of these suspects were picked up on the battlefield by militias, not by “CSI: Raqqa.”

So, what is the nature of the evidence against the detainees, and how was it collected? Are there reliable witnesses? Is there an unbroken chain of custody?

And on what legal basis can a Kurdish militia—not a fellow government, with which there is either a legal treaty or formal agreement—extradite a detainee to another country?

Sometimes, the jihadis make it easy for the state to prosecute. For example, Oussama Achraf Akhlafa was photographed next to a crucified body, boasted about being a sniper, and disseminated images of beheaded Kurdish fighters. He was imprisoned by the Netherlands for seven and a half years.

Jennifer W., facing trial in Germany, was caught talking in a bugged car about her role in the hisbah, the ISIS morality police—but only after she had managed to make her way back to Germany and lived there for two years without being charged.

These are the exceptions, however, not the rule. 

Take a case from the United Kingdom. In January, a couple turned up at Manchester Airport having just spent four years living in Syria. They had moved there after ISIS had declared its caliphate. They were arrested, and the police seized three separate electronic devices in their possession.

The problem? The couple replied “no comment” to every question asked by police. Their electronics were password-protected, and police could not access them. The investigation is ongoing, but the accused ended up being released on bail.

What’s more, this was one of the simpler cases: The couple had made it to Turkey, so they were returning with authorities’ knowledge.

A rejoinder would be: If prosecutions are not viable, why not just place terrorism suspects under surveillance?

The problem is one of resources. As of March 2017, there were 23,000 Islamist terrorism threats on the U.K. intelligence radar. And in France, the head of the domestic security agency said that about 18,000 radicalized terrorism suspects were living in that country.

That means the scale of the threat is far greater than the resources the state has to combat it. Domestic intelligence agencies already have to ruthlessly prioritize which suspects to monitor. Throwing hundreds of returnees from the caliphate onto the pile only makes that task all the more difficult. 

Hence the impasse.

The U.S. desire for Europeans to take their foreign fighters back is understandable. And Europe, not the U.S., should bear the primary responsibility with coming up with a solution about what to do with their jihadis.

However, European countries are also in a difficult situation. Accepting back hardened terrorists without much prospect of successful prosecution is an unappealing offer for any government.

The latest twist is that media outlets report the Trump administration has suggested Turkey will take control of the fates of these ISIS detainees.

Trump tweeted that Turkey “must, with Europe and others, watch over the captured ISIS fighters and families,” suggesting a broader effort is envisaged. However, details remain scarce.

So, too, do viable solutions.