Repatriating Isis fighters is a complex issue – but it’s not one that Donald Trump can solve

Donald Trump has declared that Britain, France and Germany should take back and put on trial its citizens who have gone to fight for Isis in Syria. The US has captured some 800 such fighters, and the president has threatened to release them if the UK and other European nations do not do so.

It is easy to see why the US is pushing this line. In December Mr Trump, against the advice of the US military leadership, decided to withdraw troops from Syria. As The Independent’s Middle East correspondent Patrick Cockburn has argued, this decision was more rational than the president’s opponents claim.

But it does create the huge practical problem of what to do with the people the US has captured when its troops leave. To hand them over to the Syrian government would have grave humanitarian consequences.

Under international law, once a conflict between nations is over, former combatants that have been held as prisoners of war should be returned to their countries of origin.

Here, however, the legal situation is less clear, for the fighters were not combatants of a formally recognised state; rather, they could be classified as terrorists – Isis itself has declared itself the orchestrator of a number of terrorist attacks around the world, even if it maintains that it should be formally recognised.

The US and France have both agreed to accept citizens suspected of Isis membership. Other countries, including the UK, have not. Under these circumstances, for the US simply to release the former fighters would be the default option.

But while it is easy to see why the US is pressing countries to accept their citizens, there are good reasons not to allow the US to dictate to other countries their approach to security.

As far as Britain is concerned, the situation is further complicated by the case of Shamima Begum, the runaway Isis bride who now says she wishes to return to the UK. She was a non-combatant, unlike the fighters held by the US.

It would be hard to argue that she should not be allowed to return while former fighters are repatriated, although whether or not Begum should actually be allowed to return is more complex than yes or no.

Ultimately two things should be paramount. The first is the rule of law; the second the security of the country.

We need a considered legal judgment as to the status and rights of UK citizens who have chosen to travel abroad and involve themselves, actively or passively, in a foreign conflict.

Each case should be considered on its merits. If a person has the right to return, and the practicalities of repatriation can be satisfied, then they should come back and, if appropriate, face charges. This should not be a political decision; it should be a legal one.

The government of the day does, however, have responsibility for the security of the country. If it can demonstrate that allowing a UK citizen who has fought abroad back into the country is a credible threat to national security, then it should not shy away from that responsibility.

It is these two principles, which may come into conflict with each other, that should govern the UK response to Mr Trump’s challenge. If we take the fighters back, we do so for legal and security reasons, not because we are urged to do so by an American president.