Philip Johnston: The EU aspires to be a state but if it can't keep its citizens safe, it is a failed one
Madrid, London, Paris. Now Brussels must be added to the lengthening list of European capitals targeted by Islamist fanatics in recent years. It is no surprise that the bombers who killed dozens in the Belgian city had links to the Molenbeek district, where at the weekend police shot and arrested the key suspect in November's Paris attacks.
The Brussels attackers will also have fought for Isil and returned home, possibly hiding among the tide of migrants arriving in Europe to avoid being picked up by police on the look-out for returnees.
Or they may just have wandered in and out of Belgium, France and the rest of continental Europe unmolested because the Schengen agreement's open borders made normal, counter-terrorist policing difficult, if not impossible.