As the U.S. prepares to eradicate a small remaining ISIS force in eastern Syria, European allies are grappling with the fate of their citizens detained in the war-torn nation. President Trump has demanded that Western countries accept Islamic State fighters and hold criminal proceedings in their home countries.

In a series of tweets over the weekend, Trump urged European countries, particularly Great Britain, France and Germany, to take in more than 800 fighters of European origin who had joined the Islamic State and were later captured by the U.S. military. European countries have been reluctant to allow those citizens to return.

"The Caliphate is ready to fall," Trump announced on Saturday, suggesting that should European Union countries refuse to take back their nationals, "the alternative is not a good one in that we will be forced to release them."

Minutes later, Trump added, "The U.S. does not want to watch as these ISIS fighters permeate Europe, which is where they are expected to go. ... Time for others to step up and do the job that they are so capable of doing."

Syrian Democratic Forces, which controls the northeast region of the country, estimates there are 800 Western men, 700 women and approximately 1,500 children detained in prisons and camps throughout the region, Le Monde reports.

But the question of what to do with thousands of individuals now returning to Europe — either as prisoners or of their own volition — has been a thorny issue for the legal systems of the former fighters, their spouses and children. And one that has grown more pressing as the Islamic State's control has continued to diminish throughout Syria and Iraq since the height of its power between 2013 and 2015.

Germany's foreign minister Heiko Maas addressed some of the complications involved in returning Islamic State fighters to the country in a television interview Sunday night, NPR's newscast unit reported. Maas noted that trials would be "extremely difficult to realize" largely due to the absence of "judicial information." As a result, he added, prisoners should only be brought back to Germany if it was certain that they would face trial.

More than 1,000 citizens have traveled to Syria and Iraq since 2013, with one-third having already returned to Germany, according to government figures.

On Monday, a spokesperson for Germany's interior ministry expanded on Maas' remarks, saying, "In principle, all German citizens and those suspected of having fought for so-called IS have the right to return," Reuters reported. However, she added, their extradition is only possible if the suspects have consular access — a condition the German government cannot guarantee "due to the armed conflict there."

In France, where the government has been very resistant to accepting French members of ISIS over the past six years, there have been contradicting reactions to Trump's demands.

According to Le Monde, Justice Minister Nicole Belloubet said the American president's tweets will have no impact on France's policies which determine the return of IS fighters on a "case by case" basis.

But, Agnès Von Der Mühll, a foreign ministry spokesperson, told NBC News that in light of the "American decision," the French government is "exploring all options in order to prevent these potentially dangerous individuals from escaping or dispersing."

It is unclear how many French nationals have been captured or are living in camps, but the Associated Press reported French jihadists made up the largest contingent of European recruits in the region.

Meanwhile, even before Trump's tweets, Britain staunchly rejected the idea of going into Syrian or Iraqi prisons or refugee camps in search of U.K. citizens who voluntarily left to join ISIS.

"I'm not putting at risk British people's lives to go looking for terrorists or former terrorists in a failed state," Security Minister Ben Wallace said, according to The Guardian, after he learned of a British woman who ran away to join ISIS as a teenager in 2015, but decided she wanted to return to Britain before the birth of her third child.

Shamima Begum has since given birth, according to her family, but her case and the national debate it has sparked illustrate the complexity all EU governments face in dealing with non-combatant jihadi brides and their children.

Begum, who was married to a Dutch Muslim convert shortly after her arrival in Syria, has expressed no regret over her decision to join ISIS, according to reports. The impetus for her recent pleas to return to England stem from her concern over the health of her baby. Begum's first two children reportedly died from illness and malnutrition.

Still, British officials are making no attempts to take her back to the U.K.

The government has opted to strip U.K. citizens who served as ISIS fighters of their citizenship rather than bring them back and attempt to prosecute them in court.

"That's because the standards of a court of law are very rigorous and it is very challenging to gather evidence that will eventually lead to a conviction," Lorenzo Vidino, director of the Program on Extremism at George Washington University, told NPR.

Even in cases where prosecutors achieve a conviction, most sentences are fairly short — between three to five years, he said. "But most people who have returned home are not charged, not because Europeans are soft, but it's a matter of not having the evidence."

Additionally, in the early years of the rise of the Islamic State not only was it not illegal to travel to Syria, but most EU countries had no laws prohibiting citizens from joining ISIS or any terrorist organization abroad, according to Vidino.

"There was only legislation that you couldn't join a terrorist group domestically," he said.

It wasn't until 2015 or later that those laws were introduced throughout several EU nations, "but they cannot be retroactively applied," Vidino said.

"So how do you prosecute those people?" he asked.

Overall, Vidino said, EU countries can't do much beyond continuing to monitor those who have returned home.

"It's frustrating for a good portion of the public, and something they don't really understand, but it's all [some governments] can do," he added.

Trump has said U.S. forces will pull out of Syria once ISIS is defeated.

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