The Schengen agreement is almost synonymous with the European Union itself. The treaty that lets people travel between 26 of the EU’s member countries without showing any documents is, next to the euro, the single most visible day-to-day manifestation of European unity in people’s lives. And it’s coming under increasing strain.

On September 13, Germany started enforcing passport checks at its border with Austria, saying the country couldn’t host any more refugees after seeing a surge that was expected to reach some 40,000 people (paywall) over the weekend. Trains between Austria and Germany were stopped for 12 hours.

Germany is well within its rights. As the European Commission explained in a statement, imposing border controls temporarily is “an exceptional possibility explicitly foreseen in and regulated by the Schengen Borders Code, in case of a crisis situation”. But events that can be interpreted as a “crisis” are becoming more common.

In the summer of 2015, as the flow of refugees has swollen, various countries have threatened to suspend the Schengen treaty. In June, France temporarily did so, imposing controls at its border with Italy, leaving hundreds of refugees hoping to head north stranded. France has done this before, too: In 2011, it blocked trains from Italy carrying an unusual influx of Tunisian immigrants. There have been claims that this action, by being focused on a particular group of people, was illegal under Schengen (pdf).

That’s no doubt why, in the wake of the 2011 incident as well as this summer, there have been calls for reforms to Schengen that would make it easier for countries to impose temporary border controls or closures. A poll in July found that majorities in several western European countries wanted the Schengen treaty scrapped. This past weekend’s heartwarming demonstrations of welcome for refugees across the continent likely represent the minority view.

But even adding only mild restrictions on Schengen could put further pressure on the system. If one country clamps down on movement across its borders, it can increase the strain on others further upstream where refugees are arriving, causing them to panic the way Germany did this weekend. Thus reforming Schengen increases the risk that the treaty as a whole will unravel.

This is particularly troubling in what is shaping up as arguably the worst summer ever for the European Union. It’s been only a few weeks since a last-minute agreement between Greece and its creditors prevented a “Grexit” from the euro zone. The election of Jeremy Corbyn to the head of Britain’s Labour party this weekend could, some think, increase the pressure (paywall) there for a “Brexit” from the EU. And nationalistic, anti-European tendencies are emerging in many of the member states. Could Germany’s temporary suspension of Schengen be the beginning of the treaty’s death rattle?

This article was originally published on qz.com.