The 13 November Paris attacks, which killed 130 people, prompted an urgent rethink of the Schengen agreement.
There was alarm that killers had so easily slipped into Paris from Belgium, and that some had entered the EU with crowds of migrants via Greece.
In 2015, the influx of more than a million migrants - many of them Syrian refugees - also greatly increased the pressure on politicians, and one after another, EU states re-imposed temporary border controls.