THE rules of the European Union (EU) enshrine the free movement of people. Freedom of movement has been a part of the European project since the 1950s. Yet Europe's borderless zone only really began to become a reality in 1985, when several members of the EU, including France, Germany, Belgium, Luxembourg and the Netherlands, met in a village in Luxembourg called Schengen to sign an agreement to eliminate all internal border controls. This "Schengen agreement", which came into effect in 1995, eliminated border checks among its members and allowed foreign visitors to travel throughout the area using one visa. Today a total of 26 states, both within or outside the EU, are members (see map). But the future of the continent’s passport-free zone is now under threat. Why? Two forces are pulling Europe apart. In the summer of 2015 an unprecedented number of migrants have arrived on Europe's shores. They tend to go to countries where they are mostly likely to be allowed to stay, chiefly Germany and Sweden. The pressure on these countries is growing—Germany expects to receive 800,000 asylum application this year, nearly four times as many as in 2014. On August 20th Thomas de Maiziere, Germany’s interior minister, warned that Germany's Schengen membership may be unsustainable unless other countries share the burden of accepting asylum seekers. Closed borders would mean that Germany could turn away people at its borders. Terrorism is also a factor. After last week’s thwarted attack on a train from Amsterdam to Paris, the Belgian prime minister, Charles Michel, called for review of the Schengen rules in a way that would allow more checks to passengers’ identity and luggage, in effect reinstating some border controls.

This is not the first time that the Schengen agreement has appeared to be in danger of fraying. In 2011, fearing an influx of North African refugees, Italy and France pushed for a review of the agreement. Earlier this year the Dutch prime minister threatened Greece with expulsion if it allowed migrants free passage to the rest of Europe. Neither eventuality came to pass.

Belgium or Germany would be within their rights to tighten their borders: the agreement allows Schengen countries to briefly reinstate border controls for reasons of national security. France put up its borders after the London tube bombings of 2005 and Portugal did it during the Euro 2004 football tournament. But if controls remain for more than short periods, they risk reversing decades of European integration. The effects wouldn't just be notional: various studies over the years have argued that the Schengen agreement led members to form closer trading partnerships, boosted both imports and exports, and drew tourists. Schengen is among the most visible manifestations of European unity; its erosion would send a powerful signal.

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The flow of migrants across the Mediterranean is a humanitarian crisis, and a crisis for Europe (April 2015)