The thin green line that runs through Sasa Kalin's pub, underneath the billiard table and past the bar with its gum-numbing fruit brandies, is where the European Union ends — and where it begins. But Europe's borders are about to change. And Mr Kalin's cosy pub, straddling Croatia and Slovenia, is on the reshaped front line.

“We're going to need a passport before we can go to the toilet,” complains Denis, a beer-supping customer. “Otherwise we'll be illegal immigrants.” An exaggeration, of course, but the locals have already dubbed the lavatory, on the Slovene side of the bar, the “EUrinal”.

From Friday, nine Eastern-bloc countries will join the EU visa-free travel area. Anyone will be able to travel from Tallinn to Lisbon without showing a passport.

But while some borders melt away, others on the new eastern reaches of the Schengen travel area will become tighter, more fortified. And the border between Slovenia, within the new Schengen area, and Croatia, outside it, is a case in point.

“It's not going to be easy,” concedes Mr Kalin, as he leans over the Croatian side of the billiard table and pots the black into a Slovene pocket. “This is an historically neuralgic spot, it's where the nerve endings of Europe come together.” The aroma of braised wild boar wafts through the pub, across the EU frontier.

Slovenia, which takes over the EU presidency in January, probably has the least to fret about. Admittedly, it has a 600km (370-mile) border with Croatia, and there has been friction; some bridges spanning the Kupa and Sutla rivers that mark the border will be locked and the crossing points closed. But there are no open blood feuds, little to upset the calm at Kalin's. Farther up the new EU frontier line, there is a crackling tension, a sense that a high emotional price is being paid to the EU. Schengen was the dream, perhaps the most enduring, of the second generation of EU architects, the likes of Helmut Kohl and Jaques Delors: to forge a pan-European consciousness through free travel.

Europe has swollen since 1985 — the original Schengen area was simply the Benelux countries, France and West Germany — and the terms have changed. From Friday, the old Iron Curtain moves 700km eastwards. To keep its new eastern frontier secure, the EU has invested more than €1 billion (£700 million) in electric fences, thermal imaging devices and hot-pursuit vehicles.

“We had four illegals in the garden last week,” says Sasa Kalin, in an off-hand way, as if referring to the boars that used to forage near the pub. “They holed up in my grandfather's old pig sty.” The fugitives included a Chinese and a Bangladeshi. They simply jumped over the gurgling Bregana river.

Close to the pub there are two sleepy Croat guards; the next are 800 metres away. In between, there is a slight bend in the river: the gateway to the EU. How many more loopholes figure in more than 4,000km of EU border that stretches from Estonia in the north to Mr Kalin's pub?

People-smuggling is changing fast, mutating before the new rigour on the Schengen frontiers. One route into the EU has been by boat, often from Africa, to the Croatian islands, then fishing boat to the Croatian mainland and through the green border to Slovenia.

There are crossing points that are little more than garden gates. But now the Slovenes are mopping up refugees within 30km of the frontiers, easy as it is to spot a bedraggled Asian, dropped off by the smugglers to fend for himself in the brisk Alpine winter.

So far the biggest menace has been seen as Ukraine, full of migrant workers who can treble or quadruple their earnings in the West. It is becoming a magnet for illegals from the Middle East; the Lebanese, Kurds and Iraqis.

The most vulnerable entry point has not been the long Polish-Ukrainian frontier but the short, rugged border with Slovakia. Little wonder that the Slovaks have identified the defence of this narrow strip as a national priority.

“Salaries for border guards have gone through the roof to head off the risk of bribes,” says a Slovak official. They have been kitted out with mountain-cross motorbikes and motorised skis. There are 868 guards along a mere 97km of border. The Minister of the Interior, Robert Kalinak, even has live CCTV pictures of the border in his office.

The point is to show themselves to be “good” Europeans, ready to fight off the hordes. The Germans and Austrians, both states that used to have “eastern fronts”, remain sceptical, concerned that the border might buckle and the current trickle of illegal immigrants turn into a flood.

The concern is not so much the physical transport of illegals — thermal imaging has become very effective — as the inability of eastern border guards to spot increasingly expert forgeries of travel documents. “The methods of counterfeiting have improved,” says Mr Jandl, of the Vienna-based International Centre for Migration Policy Development. Yet the expansion of Schengen involves far more than modernised customs services to protect nervous Westerners. It raises, above all, questions of identity.

The Russian minorities in the Baltic states will be cut off from the Russian motherland, or at least find travel more complicated. Will that make them more, or less, Russian? Will their disgruntlement be exploited by nationalists in the motherland?

There are already terrifying stories emerging from the Russian enclave of Kaliningrad, surrounded by the two EU Schengen states of Poland and Lithuania. The train is sealed as it crosses from Russia, but some Chechens have smashed windows to throw themselves out and claim asylum in the EU.

That, in turn, prompts questions about how the new Schengen countries, from which citizens used to try to flee to the West, will adapt to being places of refuge, leading destinations for asylum-seekers. From Slovenia to Estonia, old army barracks have been adapted for refugees, but it is still a strange, new sensation.

A diplomat in Ljubljana talks of Slovenia becoming a “bottle-stopper for the Balkans”. If violence again erupts in Kosovo, there could be a tidal wave of refugees to the EU, much as there was during the Bosnian war.

The fact is that, since the fall of communism, the eastern borderlands have grown together and now they are to be separated. The Hungarian minority in Romania has flourished since the shooting of Nicolae Ceausescu. Now, at least until Romania joins the Schengen group some time after 2011, there is again a serious border dividing them.

Ukrainians have become the servant class as the Poles migrate westwards in search of more lucrative earnings. That will dry up. Poles are looking forward to driving without impediment to their friends in Germany — that is a big step forward within the EU. And yet the price is high: the growing together of Poland and Ukraine, now interrupted, was one of the great unsung stabilising elements in Europe over the past five years.

The enlargement of the Schengen area should make Western Europe feel more secure. It has a buffer zone against future instability in Russia, the likely fallout from the end of the Lukashenko dictatorship in Belarus, the westward march of refugees if Kosovo flares up. Instead, there seems to be a sense of unease. It is little wonder that Brussels has been throwing cash at the new border policemen.

As we leave Mr Kalin's pub, on our way to inspect the heavy-duty lorries piling up on another section of the Slovene border, two unmarked Skoda Octavias block our car. A Slovene cop explains that taxi drivers have been picking up illegals and taking them across the country to Italy. And we looked suspicious, unshaven.

“New cars,” says our driver, trying to ease the tension. The Skodas are indeed sparkling new.

“The EU,” shrugs the policeman. He returns the passports. New cars, he seems to be saying, new frontiers, new world. He did not look very happy.

Why Schengen?

— The Schengen treaty is named after a village in Luxembourg where France, Germany, Belgium, the Netherlands and Luxembourg made the first agreement to cut border checks in 1985

— It was chosen because it lies at the borders of Luxembourg, France and Germany. The treaty was signed on a boat on the River Moselle, which runs through the village

— It has a population of 400, no hotel, about 36 winegrowers, a castle where Victor Hugo once stayed and a lot of petrol stations

— Visitors regularly fill up on Western Europe's cheapest petrol

— Since 1985 it has acquired a Europe Museum, Europe Square, Schengen Treaty Square and a treaty monument. The EU flag flies over its streets

— Last year, the municipality to which Schengen belongs was renamed Schengen in recognition of the village's fame

Sources: luxembourg.co.uk ; ec.europa.eu ; Times archives; magazine-deutschland.de

Times Recommends

• Thousands of Tamils dying in 'welfare camp'
• Jackson 'rebuilt his nose before he died'
• Renewed Iran protests crushed by militia