No one is exactly sure how such a process would unfold. There are no written rules to address how a country would exit the eurozone.

It’s often said the EU is like a bicycle: it must move forward or else it will topple over. As the crisis over the euro spreads, the bicycle has all but come to a crashing halt and is tilting dangerously to the side.

As bad as the continent’s debt woes are, Europhiles are even more concerned about the cracks appearing in the most fundamental element of modern Europe: unrestricted movement.

While the call by Guben’s mayor for border controls has fallen on deaf ears in Berlin, in other parts of Europe barriers are already going up. Denmark re-established checks at its land border with Germany and at its bridges and seaports in July with the stated goal of stopping illegal immigrants and criminals who have breached the outer edges of Europe’s border-free zone. Under the Schengen agreement, signed in 1985 and of which Denmark is a signatory, members can only impose controls at inner borders on a temporary basis “in the event of a serious threat to public order or national security.” No such threat exists, yet customs officials have been pulling over random cars. Justifying the new restrictions, Danish Finance Minister Claus Hjort Frederiksen alleged: “We have seen too many examples of violence, break-ins and brutal criminality committed by perpetrators who have crossed the borders.”

The reaction from the rest of Europe has been quick and hostile. Germany’s minister for justice and the EU, Jörg-Uwe Hahn, urged Germans to vacation in Poland or Austria rather than Denmark. Danish tourism agencies, which usually profit from renting summer cottages to Germans, have reported cancellations and complaints.

Charles Kupchan, a professor of international affairs at Georgetown University, describes Denmark’s decision as part of a broader “re-nationalization of political life that is sweeping Europe.” It’s driven, he says, by continent-wide concerns about immigration, the debt crisis, and a lack of leadership from traditional EU powerhouses, particularly Germany. “The borders of EU member states are effectively coming back to life,” he says. “The developments of the last three to five years raise very troubling questions about the project of European integration. For the first time, it is reasonable to question whether Europe has reached its high-water mark and will either go no further, or will slide in reverse.”

The first signs of trouble for European integration didn’t start with the economic crisis. Kupchan points to failed referendums in France and the Netherlands in 2005, in which voters overwhelmingly rejected the European constitution. Yet the debt crisis has exposed the limits of European togetherness and put member nations on a collision course in ways not seen in decades.

In Germany, resentment is growing at the prospect of more bailouts for spendthrift Mediterranean Europe. Earlier this year, the German newspaper Bild published a photo of a Greek banner that labelled German Chancellor Angela Merkel and French President Nicolas Sarkozy “Nazis,” complete with the yellow stars of the EU flag rearranged into a swastika. The headline blared: “We pay—still we are abused!” Meanwhile, a new survey in the Netherlands found the majority of Dutch (54 per cent) want Greece ejected from the EU rather than continue the bailouts. Fully 60 per cent of respondents said the Netherlands “should stop lending money to other eurozone countries now.”