After the Brussels airport atrocity, the Europeans need to tighten things up. First off, they need to catch the perps, and most important, once caught, put them on trial and convict them of premeditated murder, and sentence them to death. And force the courts to get it done within six months from the arrest. And actually carry out the death sentence, not take ten years of appeals and welfare for lawyers. The deterrence effect of conviction and punishment wears off over time. When the courts take ten or twenty years to render a decision, nobody cares. I can't get excited about a ten year old crime. Too much time has passed. But I can get excited about a fresh horror.
BTW, US courts need to get on the stick too. For instance they haven't done a thing about Tsarnaev, the Boston Bomber and it's been three years.
Second, Euro cops need to develop some sources, informants, who can tip them off to bad guys setting up to do bad. The cops need to be where the perps live. They cannot write off the Molembeeks and the banlieus as "too tough to bother with". You gotta have cops, on foot patrol, making contacts, and developing informants in the places that the bad guys live. You start with the shop keepers and the landlords. Let them know that hordes of government inspectors, tax audits, and other official badness will descend upon them if then don't become informants, and finger bad guys. Every small time criminal they catch, is offered a choice between cooperation with the cops, or prosecution. A few months of this will develop some sources.
Then the EU needs take real military action in Syria. Like send in an army, occupy the place, blow Assad and his government away, execute what ISIS people they catch. Set up decent law enforcement, fire protection, and get the schools open and operating. Make sure they are real schools and not Islamic madrassahs. Do land reform, namely dispossess the landlords, and give the land out to the farmers who actually work the land. Give those farmers good titles to their land, and make sure the courts enforce the new titles. Set up land offices to record exactly who owns what. Fix up the irrigation systems, canals, reservoirs, dams, pumps, locks, and such. Get the local industry back into business, hiring people and paying wages. Give military protection to any place that employs people (out side of family members) Set up some banks to finance business. Make Syria a decent place to live, with jobs and a rule of law and then the Euros won't have all those Syrian refugees swarming into Europe.
This might take a few years and cost a bundle, but it's worth it.