Riot police face protesters during an April rally against the Austrian government's planned reintroduction of border controls at the Brenner Pass on the border of Austria and Italy. (Jan Hetfleisch/EPA)

Since the days of ancient Rome, conquering armies have traversed the Brenner Pass, a scenic gorge in the Alps connecting the boot of Italy to the heart of Europe. Now, nations to the north fear that this vital passage will become the funnel for a new “invasion” of migrants.

A thousand miles away in Greece, the main migrant route into Europe is shutting down amid stricter border controls in the Balkans and a deal with Turkey to stop new arrivals from the Middle East, Africa and beyond. Yet as one door closes, concern is mounting in a host of countries that the poor and desperate may find another way in.

Claiming that as many as 1 million more migrants are massing in Libya with the aim of crossing into Europe through Italy, the Austrians, for instance, are laying the groundwork for an emergency fence between the jagged Alpine peaks at its Italian border. To stop the feared hordes, the Swiss are threatening to call out the army (yes, Switzerland has an army). The Germans and the French, meanwhile, are joining an effort to extend “crisis” checks already in place at various European Union borders despite early signs that the region’s migrant flows may be coming under control.

The Brenner Pass, a narrow border crossing in the Alps between Italy and Austria is the latest tension point in Europe’s migrant crisis. Protesters there are demonstrating against Austria’s proposed increase in border controls. (Zoeann Murphy/The Washington Post)

Some Austrian politicians are backing a possible fence at the Brenner Pass despite their historic ties to Alto Adige, a still largely German-speaking enclave in Italy ceded by Vienna in the early 20th century. At local restaurants, schnitzel competes with pizza on menus, and families of Grubers and Hubers outnumber Rossis and Bianchis.

[Timing not yet right for Libya intervention, Italy says]

But if a fence can hold back migrants, some Austrian politicians say, then a fence there should be.

“We can’t be the social security for Africa,” said Rudi Federspiel, a regional leader in the right-wing Freedom Party of Austria from the bordering province of Tirol. “Most of these people are Muslims, not Roman Catholics.”

Those migrants already in Austria, he insisted, are causing serious social problems: “We have rapes. Rapes in the city. Rapes all over the place. Because [Muslim] men don’t accept women. . . . They are not on the same level” as Europeans.

In 2014 — before migrants started choosing the easier route via Greece — Italy was the ground zero of Europe’s migrant crisis. Already, hundreds of migrants per week — most of them sub-Saharan Africans who first arrived at ports in Italy’s south — are again seeking to venture north through this majestic valley.

So far, overall arrivals to Italy — about 28,600 since Jan. 1 — are roughly on par with 2015, and are not yet near the huge numbers seen in Greece at the peak of the crisis last year. Nevertheless, they are provoking what Italy calls a disproportionate, even hysterical, response from its neighbors. This week, Italian Prime Minister Matteo Renzi suggested that the ugly tone of the fence debate with Austria in particular risked digging up dark chapters in European history marred by divisions and bigotry.

“If you play on fear, you risk giving strength to those who know best how to wake the ghosts and specters of the past,” he said.

Amid a growing migrant backlash, the rush to bar the doors in some nations is indeed being framed in populist — some say xenophobic — terms. Last week, the Swiss village of Oberwil-Lieli voted in a local referendum against accepting 10 refugees, despite having to pay a 300,000 euro ($345,000) fine for the privilege.

Last month, Austria passed a law allowing mass rejections of asylum requests in the event of a huge new wave of migrants. A gang of right-wing youths recently stormed the prestigious Burgtheater in Vienna, spilling fake blood onstage and interrupting a play about xenophobia. In March, a pub in the southern Austrian town of Althofen banned all refugees, posting an open letter to them saying that other patrons “don’t feel comfortable when you are in the bar.”

The Austrian police have started random checks and have broken ground on a patrol station, where blanket inspections could rapidly be introduced along one of Europe’s busiest corridors for commerce.

The Italians have refused Austrian requests to board trains heading north to stage migrant hunts. Austrian police, meanwhile, have deployed tear gas against Italian protesters resisting the border action in recent weeks. On Saturday, hundreds of demonstrators again mobilized against the Austrian plans, some hurling smoke bombs and stones at Italian riot police. Italian business owners and politicians are warning that large-scale checks could cost the regional economy up to $3.45 billion a year in shipping delays.

[U.N. says up to 500 migrants may have drowned on their way to Italy]

If a massive wave of migrants does come, local politicians say they will be distributed to communities across the region rather than housed in one big camp if they cannot cross the border. But some in this quaint community of gingerbread-house-like villages and Alpine ski resorts are fretting that their clean, quiet streets may turn into a “new Idomeni” — a reference to the squalid refugee camps on Greece’s sealed border with Macedonia.

“The population is scared because they see all the footage from Greece . . . and fear this might happen here, too,” said Elmar Morandell, transport chief for the region’s Association of Trades and Services.

Any fence, Austrian officials say, remains a contingency — a barrier that will go up only in the event of a major migrant surge. But on a continent where freedom of movement and open borders became the linchpin of the E.U., critics say that such moves are part of an effort to effectively fence off Europe’s south, ensuring that if large numbers of migrants again arrive this year, they would be largely corralled at entry points in countries such as Italy and Greece.

The Austrian response, observers say, is partly an outcrop of roiling domestic politics there. Vienna’s ruling coalition is under intense pressure by the right-wing populist Freedom Party of Austria. Norbert Hofer — a Freedom Party politician who said he understood why more Austrians are buying guns amid the refugee crisis — staged a spectacular first-place finish in last month’s presidential elections there, strongly positioning himself ahead of a May 22 runoff vote.

Austria has now adopted a hard line on new migrants. But Hofer has called for even tougher measures, including stricter border controls. He mocked the barrier — a chain-link fence with no barbed wire – that Austria has erected at its border with Slovenia as a “garden fence” with holes in it.

“This is no border protection,” he said.

In an interview, Italian Foreign Minister Paolo Gentiloni called for politicians to stop confronting the region’s migrant crisis with populism.

“We have to work on this, probably for the next months and years, without giving the impression to our public opinion that . . . we have to resist a barbarian invasion or that we have boats filled with terrorists,” he said. “All these kinds of stories are not true, and very dangerous.”

And yet, Italy is also worried about a migrant surge, and is pressing for the start of a broad NATO mission off the coast of Libya in July, similar to one patrolling for human smugglers between Greece and Turkey in the Aegean Sea. In an apparent effort to thwart the Austrian plan, Italy also has vowed to redouble its efforts to stop irregular migrants from venturing north.

On a recent evening in Brennero, Italian police stopped about two dozen migrants attempting to board trains going north over the course of 90 minutes. Awil, a 16-year-old from Somalia, sat dejected at the station, clutching his head in his hands as several uniformed Italian police held him and a group of his friends in a room.

He said he had landed in Sicily on April 12 in a bid to make it to Germany. He claimed the spot checks here were mostly targeting Africans.

“They only stop people who are black, like me,” he said. “They let Syrians into Europe, why? We are suffering, too. It is not fair. I tell you, man, it is racist.”

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Today's coverage from Post correspondents around the world

Stephanie Kirchner in Berlin and Stefano Pitrelli in Rome contributed to this report.