In a perfect world, where all economies were equal, if everyone had roughly the same amount of money, where people valued their own culture and wanted to stay in countries they were born in and critically where bad people did not want to roam widely to harm others, unrestricted free movement of people might have worked.
We can clearly see it does not. It is not a perfect world we live in.
What is it?
An agreement, signed in 1985 in the town of Schengen in Luxembourg, to remove border checks within Europe. It means anyone, regardless of nationality, can move freely between member states without showing a passport or visa.
Who is a member?
Not the UK. But most EU states are in, as are Switzerland, Iceland and Norway. In total, 26 countries comprising 400 million people.
Why is it under strain?
Terrorists and mass migration. Police checks have been brought in on the Italian border at the request of Bavaria, amid a wave of non-EU migrants attempting to reach Germany. Angela Merkel warns the system will be pulled apart unless countries share asylum seekers. And Belgium wants more ID checks on trains in the wake of the Thalys train terrorist attack.
Are checks legal?
Police are allowed to make targeted ‘security’ checks on the border, and states can impose border controls in an emergency or for major events for up to 30 days. But permanent, systematic checks on passports are forbidden.
What does the EU say?
Jean Claude Juncker, the European Commission president, says the system is non-negotiable, irreversible, and the EU’s greatest achievement.
What do Eurosceptics say?
“Schengen has now hit the buffers of the real world and is falling apart,” says Nigel Farage, Ukip leader.