You can always rely on European bureaucrats to give something a dull and dry name. The Schengen agreement, as the plan to do away with Europe's shared border controls later became known, was named after the small town (population: 4,313) in Luxembourg near where a handful of European nations met on a riverboat in 1985.

While that agreement may not sound exciting, what it resulted in was virtually unprecedented in its ambition. There are now 26 different countries within the Schengen area. In total, a whopping 400 million people can travel freely in an area which spans over 1.6 million square miles. For many Europeans, the Schengen zone has a remarkable symbolic power: Remember, not so long ago this was a continent that was physically divided by walls.

Thirty years later, however, it's clear that the agreement has had some unintended consequences and now faces serious criticism. So what exactly is the Schengen zone? And why is it in trouble? Let us explain.

How was the Schengen area created?

Article 48 of the 1957 Treaty of Rome had enshrined the principle of the "free movement of persons" in the European Economic Community (EEC), a predecessor to the European Union. However, member states bickered about the practicalities of what this actually meant for decades.

It was only when Belgium, France, Luxembourg, the Netherlands and West Germany met on that riverboat in 1985 that an agreement for the gradual abolition of border controls in Europe was reached. The 1985 Schengen agreement was followed by the 1990 Schengen Convention, which cemented the idea of a common visa policy.

After years of yet more debate, the Schengen agreement was finally implemented in 1995. Originally conceived of as separate from E.U. law due to disagreements, it became incorporated into E.U. law in 1999 when the Amsterdam Treaty came into force.

Who is currently in the Schengen area?

There are 26 countries in the Schengen area, including all E.U. nations except for six. Britain and Ireland opted out of the agreement at its inception and are not required to join. Four states which recently joined the European Union – Bulgaria, Croatia, Cyprus and Romania – are legally required to join the Schengen zone later and a number of non-E.U. states, including Norway and Switzerland, have agreed to join through separate agreements.

What are the positives of the Schengen area?

E.U. officials often celebrate the apparent success of the Schengen agreement. Writing in French newspaper Le Figaro last month, Jean-Claude Juncker, president of the European Commission, said that the Schengen area was "one of our very greatest achievements."

Outside of the obvious symbolic importance of the system, there are clear practical benefits. The system helps trade between the nations, and it means that a tourist could travel from Poland to Portugal by train without pulling out their passport. It's helped Europeans lead increasingly interconnected lives: it's not unusual for people to live in one Schengen area country and commute to work in another one, for example.

So, do Europeans support it?

Not exactly. In recent years a number of polls have shown convincing opposition to the Schengen zone in a variety of nations. In 2012, for example, a poll by Ipsos MORI found a majority of citizens in France (64%), Belgium (62%), Italy (62%), Sweden (59%), Spain (54%) and Germany (51%) supported the reintroduction of border controls. A poll published earlier this summer in the French newspaper Le Figaro suggested a similar level of opposition remained.

Wait, why would they oppose it?

When European states first envisaged the Schengen area, they imagined that while internal border security would be relaxed, European nations should work together to considerably strengthen their collective external border. The problem is, that's easier said than done. As conflicts in the Middle East and elsewhere left many people displaced over the past few years, migrants and refugees have headed toward Europe at a rate faster than anyone had anticipated. Despite attempts to find a shared strategy for securing Europe's external borders, many get into Europe.

When the current migrant crisis began, many people were arriving in Italy after making perilous journeys across the Mediterranean by boat. More recently, there has been a surge in people reaching Greece after crossing from Turkey and then travel through the Balkans until they reach Serbia or another Schengen zone country. Wherever they first arrive, many of these migrants and refugees hope to eventually journey onward to Western Europe where they may be more likely to have an asylum claim accepted. Germany is widely expected to receive far more migrants than any other country in Europe this year, for example.

It's a situation that can lead to disastrous consequences, as made clear by the recent discovery of 71 dead people, presumed to be migrants, in the back of a truck in Austria. And countries at the periphery of Europe, often poorer countries such as Greece or Hungary, often face the worst of it: Not only do they bear the brunt of the arriving migrants, thanks to E.U. legislation known as the as the Dublin procedure they can also be expected to review their asylum claims even if the migrant travels onward to another country.

How are these states reacting?

Hungary, currently led by the conservative government of Viktor Orban, made international headlines for a variety of measures clearly designed to stem the flow of migrants through the country. On Tuesday it shut down its main train station after it became overrun with people hoping to reach Western Europe. It has also built a 109-mile fence along its border with Serbia and briefly attempted to skirt its Dublin procedure obligations.

Many of these moves appear to run counter to the spirit of the Schengen agreement, and Hungary is far from the only government making moves that question a borderless Europe. Denmark's government recently announced that it planned to reinstate controls along its border with Germany, while a French court ruled earlier this summer that border controls aimed at stopping migrants crossing from Italy were legal. Just last week Austria has significantly increased the number of inspections on its borders, leading to backups of trucks that stretch for miles.

Article 2.2 of the Schengen treaty does allow for a brief reinstatement of border controls for "public policy or national security" reasons, but many supporters of the Schengen area are worried that this represents an existential threat to the agreement.

Is immigration the only issue with the Schengen area?

No. While polls do suggest that most critics of the Schengen area view immigration as a prime reason for restoring border controls, there's also the intertwined problem of security. This issue was highlighted last month when an attack on a train between Amsterdam and Paris was thwarted by a number of passengers. The suspect in the attempted attack, Ayoub el-Khazzani, was known to European intelligence services but had bought his ticket in cash and had not been required to show identification to board the train.

Adding to the concerns are reports that not all countries adhere to the same security standards when travelers arrive from outside the Schengen area. After the attacks in Paris in January, Gilles de Kerchove, the E.U. counterterrorism coordinator, admitted that more than half the time border guards simply checked E.U. citizens' passports to judge if they were authentic rather than putting them in a computer to see if the passport was in any databases.

Another considerable factor is that illicit goods, including weapons, can be transported across borders in the Schengen area with considerable ease. Experts say that many weapons found in Western Europe, including those used in January's Paris shootings, were likely smuggled from former conflict zones in Eastern Europe.

If it really is broken, can it be fixed?

European officials are scheduled to meet on Sept. 14 to discuss how they could better deal with asylum seekers and other migrants. Some nations, including France and Germany, have proposed a quota system to ensure that refugees are shared more evenly between European nations. The hope is such a plan could help relieve some of the problems associated with the Schengen area. "If we don't succeed in fairly distributing refugees then of course the Schengen question will be on the agenda for many," German Chancellor Angela Merkel said at a press conference in Berlin on Monday.

These sort of warnings have been heard many times before – by then-French President Nicolas Sarkozy in 2012, for example – and the Schengen area has endured. It may well endure again this time, especially if European nations can find a way of better coordinating their asylum and security systems.

However, the criticisms of the Schengen system and the European integration it represents are unlikely to go away anytime soon. All across Europe, anti-E.U. political parties have made steady gains over recent years. Many of these parties and the European voters who support them see the Schengen area, much like the euro, as an example of a naive European bureaucracy that stretched itself too far.

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