As Germany enforces tougher checks on all its borders, Andrew Mueller questions what this will mean for the Schengen Area. See omnystudio.com/listener for privacy information.
In 1985, five countries France, Germany, Belgium, the Netherlands and Luxembourg signed an agreement in the settlement of Schengen, symbolically located where Luxembourg's border intersects with those of France and Germany.
It was the inauguration of an era in which residents of and visitors to continental Europe have become accustomed to coming and going across land borders, in particular as they please, without being halte at forbidding checkpoints and asked by querulous sentries for their passports, papers or the purpose of their journey.
All 27 members of the EU, other than Cyprus and Ireland, are now members of what has become known as the Schengen area, as are Iceland, Norway, Switzerland and Liechtenstein, officially, and the microstates San Marino, Andorra, Monaco and the Vatican.
Effectively, Schengen is the definitive manifestation of the European dream, an eutopia, if you will, a continent united by free movement and frictionless trade, to the point that it scarcely seems remarkable that people might live in one country and work in another.
In Luxembourg, actual and spiritual home of Schengen, perhaps half its workforce commutes daily from a neighboring nation.
As more than one utopia has discovered, however, such delirious visions have a way of getting rumbled by reality.
Starting next week, on September 16, for at least six months, Germany will resume checking the credentials of new arrivals.
Germany had already imposed controls on its land borders with Poland, Austria, the Czech Republic and Switzerland.
It will now extend these measures to all its other land borders, that is, those with Denmark and its fellow OG Schengen states of France, Belgium, the Netherlands and Luxembourg.
The reason for this is a long simmering issue which has been brought recently to the boil.
There was always an inbuilt shortcoming with Schengen, which was that from one day, week, month or year to the next, its member states would have no real idea who is on their territory.
This flaw was magnified by the colossal refugee inflows of 2015, when columns of hundreds of thousands of people, mostly but not exclusively from war, ravaged Syria, yomped across Europe in search of somewhere better.
Germany was always going to be the most attractive destination.
It is big, prosperous, had established migrant communities, and was also, for obvious enough historical reasons, squeamish about being seen to take a hard life.
Then Chancellor Angela Merkel made a practical and generous declaration.
Wir schaffen das.
Roughly, we will manage.
It was no trivial undertaking.
In 2015 alone, 1.1 million asylum seekers arrived in Germany, almost an entire new Munich.
And those were just the ones anybody knew about.