The Paris attacks and the subsequent pressure on France to tighten its borders have put the future of the Schengen Area of passport-free travel at risk.
The Schengen treaty allows travellers to move through most EU countries without having their passports checked or having to go through border checks.
The policy makes to it possible travel from Spain through to Finland without being stopped.
However, EU governments have faced pressure internally to implement tighter checks after the attacks on Paris, which left 130 people dead.
It has been reported that at least two of the attackers came to the continent with refugees entering Greece before crossing into Belgium and France.
As the flow of refugees continues across the Balkans and into EU territory, European leaders could find themselves ending what was once a cornerstone of EU policy.
Al Jazeera’s Laurence Lee has more from Slovenia and Croatia.