The two-decade-old era of border-free travel in Europe was unravelling on Monday as countries imposed controls on their frontiers in response to an unprecedented influx of migrants.
Germany's surprise decision to restore border controls on Sunday had a swift domino effect, forcing neighbours to shut their own frontiers as thousands of refugees pressed north and west across the continent.
Austria dispatched its military to guard its frontier with Hungary after thousands of migrants crossed the border on foot overnight, filling up temporary accommodation space in tents and railway station car parks.
"If Germany carries out border controls, Austria must put strengthened border controls in place," Vice Chancellor Reinhold Mitterlehner told a joint news conference with Chancellor Werner Faymann. "We are doing that now."
He and Faymann said the army would be deployed in a supporting role.
"The focus of the support is on humanitarian help," Faymann said. "But it is also, and I would like to emphasise this, on supporting border controls where it is necessary."
Slovakia said it too would shut its own borders with Austria and Hungary.
Biggest threat to Schengen
The measures were the biggest threat to the Schengen system, which since 1995 eliminated frontier posts across Europe and ranks alongside the euro single currency as one of the transformative achievements of integration on the continent.
The 26 European countries in the Schengen area issue common visas and leave the borders between them unguarded. Frontiers which were fought over for centuries and which choked off traffic and trade just a few years ago, are now marked by little more than signposts on highways across the world's biggest economic bloc.
But the rules still bar undocumented migrants from travel within the zone, while leaving few mechanisms to stop them.
That has created chaos as hundreds of thousands of people, including refugees from war in the Middle East, arrive on the bloc's southern and eastern edges and head to richer and more welcoming countries further north and west to seek asylum.
European interior ministers were holding crisis talks, with Germany, France and the bloc's executive Commission trying to overcome opposition from eastern members to a plan to relocate 160,000 refugees from Italy, Hungary and Greece.
Poland said it was prepared to impose controls if migrants aimed for its frontier in large numbers, and any EU decision to impose quotas for accepting refugees on member states without their agreement would lead to institutional crisis.
"We will accept only as many refugees as we can afford, not a single one more or less," said Prime Minister Ewa Kopacz.
A draft agreement would also include strong language on the need for tighter controls of the bloc's external borders, rapid screening of arrivals and deportation of those without valid asylum claims, to help assuage countries concerned that relocating asylum seekers could attract more people.
Countries within Schengen are permitted to reimpose border checks on a temporary basis in emergencies, and have occasionally done so in the past, but not on the scale unfolding on Monday.
Most of the refugees have been bound for Germany, which announced in August it would suspend EU asylum policy to accept Syrians who arrive elsewhere in the EU, creating greater incentive for migrants to trek across the bloc.
Austria had shuttled refugees directly on to Germany. But since Germany announced border controls on Sunday, migrants walked across the border into Austria from Hungary at the fastest rate yet, without being able to travel onward. An Austrian police spokesman said in the early afternoon that 9,000 people had arrived since midnight, after 14,000 on Sunday.
"The accommodation centres in Nickelsdorf, Parndorf, and in the near surroundings are all full," the police spokesman said, referring to the area near the main border crossing.
"At the moment, no buses are running," he said. "The only buses that are running are taking people to the station until a special train leaves, but otherwise we have no accommodation at the moment."
Germany's Chancellor Angela Merkel has taken a lead on the issue, announcing that Europe's richest country was able and willing to host hundreds of thousands of migrants and preparing for as many 800,000 asylum applications this year. Her vice chancellor said in a letter to party members seen by Reuters that this figure could rise to 1 million.
She has also demanded that other EU countries do their part by accepting some refugees who would be relocated from the main countries where they arrive — Italy, Greece and Hungary.
EU Commission President Jean-Claude Juncker has proposed a formula to distribute refugees based on EU member states' economic strength and population. Countries would be allowed to contribute money to escape their quotas.
But the measures are strongly opposed by eastern European countries, including Poland, Slovakia and Hungary itself, even though Hungary would be one of the three beneficiaries.
French President Francois Hollande, who has joined Merkel in campaigning hard for binding EU-wide quotas, called for rapid agreement on controls of entrants along the EU's external borders as well.
"In concrete terms that means putting registration centres in Greece, Italy and Hungary," Hollande said as EU interior ministers met on the matter in Brussels.
"It must be sorted out today."