Anis Amri, the man thought to have carried out the recent terror attack on a Berlin Christmas market, was able to travel through Europe without triggering any alerts. He took long distance buses and trains to travel first to the Netherlands, then France, and finally Italy. Such unchecked movement is made possible by the Schengen zone, the EU's borderless travel region. But even the Schengen zone is not as unfettered as it once was. Since the height of the migration crisis in the fall of 2015, several states have reintroduced border checks. Germany, for example, now conducts checks along its border with Austria. Borderless travel is one of the core European ideals, and something that no state would give up lightly. But in this era of terrorism and uncontrolled migration waves, Schengen is being put under increasing pressure.

The EU has already decided to check air passenger data in future. After around five years of negotiations about data protection, the European parliament voted in April 2016 to introduce an EU-wide system. Airlines will be required to save passenger names, phone numbers, payment methods, seat reservations and other information given by passengers into a common system, so that the relevant security authorities can more easily trace suspects. The system will first be introduced for flights into and out of the EU, but member states can also use it for flights within Europe.

Larger practical problems

Belgium is concerned, however, not just by the sluggish introduction of the system, but by the fact that it only applies to air travel. It makes no sense to close one door "and leave all the others open," said Belgian Interior Minister Jan Jambon. Belgium is planning to collect and evaluate data on passengers on all international trains, buses and ships. It is also in talks with neighbors France, Germany, and the Netherlands. The goal of the Belgian government is the EU-wide collection of such data.

But its proposal is getting an unenthusiastic response, particularly from rail companies. In September, the European rail association CER warned that across-the-board passenger checks would have a severe impact on rail companies and result in a decrease in passenger numbers. A member of the European Commission also voiced "particular concern" that cross-border travel would be interrupted. If checks were to be conducted in train stations, for example, long lines would form, presenting terrorists with "new targets."

But there are also those in favor. The Dutch newspaper "Volkskrant" quoted sources in the justice ministry in The Hague as saying that the government under Prime Minister Mark Rutte is working on a proposal whereby passengers booking tickets on international trains and buses would be required to show ID at the time of purchase. Marine Le Pen, the head of the French far-right National Front, has gone much further. "This escapade across at least two or three countries is symptomatic of the total security debacle that is the Schengen zone," she said about Amri's flight following the Berlin attack.

Sweden leading the way

Even if Belgium were to bring more allies on board, the plan throws up a host of questions, in particular: What about the road traffic that makes up for the bulk of cross-border activity? Belgian border crossings are already fitted with cameras that record license plate numbers, but a car carrying a potential terrorist would have to be registered as suspicious. In the case of the Berlin attack, a warrant for Amri was first issued 48 hours after the attack, giving him enough time to cross several borders. Adding to the complexity: Amri, like many other terror suspects, was traveling with fake passports and using several identities.

But we can already study what happens when mandatory checks are implemented: For exactly a year now, Sweden and Denmark have been checking everyone crossing the Oresund Bridge between Malmo and Copenhagen by car, train, or ship. Not so much to defend against terrorism, but to control the flow of migrants to Sweden. This has had a considerable economic impact. The time-consuming border checks mean some commuters now spend hours traveling to and from their jobs in the neighboring country. They have even formed a group that is now demanding financial compensation for the losses they've incurred as a result.