But the perks of free-movement may be under threat. There's been increase in the number of terror attacks by suspects who've dipped in and out of intelligence service radars as they move across, in, and out of the EU. A recent foiled attack on a train from Paris to Amsterdam was carried out by Ayoube El Khazzani - a 25 year old Moroccan national with a Spanish passport who boarded a train in Brussels carrying several weapons. He'd also been to Turkey and Syria but flew there from Germany.

　　The EU is fighting calls to put up border controls and passport checks on train services. Natascha Bertraud is a spokeswoman for the European Commission.

　　欧盟难民危机的影响

　　"It's very clear that the Schengen area is not and has never been the problem., Security checks on trains is perfectly allowed just as they are on airplanes but of course the EU not being a police force or intelligence service, security primarily remains a national prerogative and responsibility."

　　And that - is part of the problem. Without a centralized EU intelligence unit, people like Ayoub El Khazzani, or the Koachi brothers who carried out the attacks on the Paris-based Charlie Hebdo magazine last January, slip through the cracks.

　　A lack of frontiers has come under scrutiny thanks to the EU's migration crisis too. Asylum rules state that refugees must apply in the first European country they arrive in. But thousands of migrants landing in countries like Greece and Italy are moving upward, to countries like Germany where they get better benefits. German Chancellor Angela Merkel says that if European countries don't sign up to share the burden of refugees - Schengen could be called into question. Pietro De Matteis is President of European Federalists Party. He says tighter border controls are not the answer.

　　"German interior minister mentioned we may need to suspend Schengen because of the current asylum seekers crisis. Clearly this is not a solution. Because asylum seekers, first of all have the right to come to Europe. And secondly by putting frontiers and controls across Europe it’s not going to stop them to come and seek a better future. "

　　Along with the euro the achievement of a borderless EU has been one of Europe's proud achievements. Both the migration crisis and threats of home grown terrorism are reviving calls for more integrated asylum policies, and more intelligence sharing. If Schengen were to fail, it would undermine efforts to deepen the European Union on every level.

　　For CRI I'm Sandra Gathmann in Brussels.