In 1985 the European Union ratified the Schengen Agreement, allowing citizens of most countries within the EU to travel across borders without a passport check. This is commonly referred to as "free movement." Some background:
The Schengen Agreement abolished many of the EU's internal borders, enabling passport-free movement across most of the bloc.
Schengen is often criticised by nationalists and Eurosceptics who say it is an open door for migrants and criminals.
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.
But like most open border policies, the Schengen Agreement has been a disaster for Europe and controlling illegal immigration from North Africa and the Middle East, overwhelming the continent. Recently, the unprecedented refugee crisis from Iraq and Syria has put European leaders welcoming them under heavy pressure and scrutiny. As Germany and Sweden continue to rack up rape statistics, citizens are becoming rightly intolerant of open border, zero-questions asked immigration policies.
If any one person drove the United Kingdom out of the European Union, it was Angela Merkel, and her impulsive solo decision in the summer of 2015 to throw open Germany—and then all Europe—to 1.1 million Middle Eastern and North African migrants, with uncountable millions more to come. Merkel’s catastrophically negative example is one that perhaps should be avoided by U.S. politicians who seek to avert Trump-style populism in the United States. Instead, the politician who most directly opposes Donald Trump—presumptive Democratic nominee Hillary Clinton—is doubling down on Merkelism.
Borders and sovereignty matter. As other countries consider leaving the EU now that Britain has led the way, they'll also consider taking their borders, sovereignty and safety back from the bureaucrats in Brussels.