Thousands of Western citizens went to the Middle East to fight alongside the Islamic State of Iraq and al-Sham (ISIS) in Syria and Iraq. Now that the “caliphate” they helped build has collapsed, some are going home.

This has occasioned much hand-wringing in the West about how to respond to this. And that’s puzzling.

Western governments should have no trouble figuring out what to do citizens who have hacked off the heads of aid workers, committed mass rapes, pushed gay people off buildings or tried to erase the presence of religious minorities.

Killing them on the battlefield is one option. If they are captured, legally detaining them is another. And if these fighters do make it home, their home countries should prosecute them. (Unfortunately, an absurdly lax legislative approach in some European states makes it hard to prosecute, usually for jurisdictional or evidentiary reasons.)

What should not be an option is to treat the returning terrorists with ever-greater levels of tolerance. Yet that is the option a number of Europeans advocate.

Take Denmark. Its much-vaunted “Aarhus model” sees returnees from Syria essentially welcomed back without censure. Danish authorities treat returning fighters as naive youths. They may be offered public housing and counseling to help them readjust to life back in Denmark.

Some EU officials, former spooks and academics are also pushing the idea that “disillusioned” returnees should serve as “countermessengers” against ISIS rather than be placed in prison. EU counterterrorism coordinator Gilles de Kerchove suggested that “some returnees who don’t have ‘blood on their hands’ are a strong credible voice for counternarrative purposes. They can explain … that they thought they were joining a nice idea of the caliphate but encountered people … being violent.”

Some of those advocating tolerance are quick to criticize governments’ tougher lines.

Certain governments have removed the citizenship of dual nationals fighting abroad in order to prevent their return.

This is a sensible measure designed to help prevent terrorist attacks in the West. Yet it has been castigated as building a pathway rather than a barrier to violence. One of the psychologists responsible for the Aarhus model said, “We’ll end up with nomadic young people having no other opportunity than criminality or violent destructiveness.”

Of course, one constituency laps up these arguments: the foreign fighters themselves. One recruit convicted in Kosovo of fighting in Syria says that rather than prison, “reintegration” is required. Another returnee, when asked about British plans to remove the passports from those fighting in Syria, told British TV that it would “only cause more problems. … They will only have more hatred in their hearts, and then they might take revenge.”

We should ignore such pleas.

It would be a grave mistake to forget that ISIS and its supporters have brought bloodshed around the world, subjugating Iraqis and Syrians and slaughtering innocents in New York, Paris, Berlin, Stockholm and countless other cities. And it would be just as big a mistake to forgive those who helped them along the way.

What is needed instead is an effective deterrent against foreign fighter travel. The convergence of foreign fighters that occurred in Syria was highly destructive — and nothing new.

That phenomenon is fast becoming the norm. To varying degrees, it also occurred in Bosnia, Afghanistan, Iraq and Somalia. European governments did not prosecute such travelers, so it will doubtless happen again — unless Western governments crack down on citizens who have joined terrorist groups.

Forgiveness, no jail, keeping your passport, council housing and the offer of counseling are hardly deterrents to foreign fighter travel. The prospect of spending the best part of your life in prison may not end up being much of a deterrent either, but it has a far better chance of working than some of the trendier alternatives.