France's Marine Le Pen with supporters. Their t-shirts feature her campaign slogan, which translates to "in the name of the people."Claude Paris / AP

Like the advance of the Ottoman Empire in the 16th century, the spread of conservative populism across Europe stalled at the gates of Vienna last weekend.

The failure of Suleiman the Magnificent to capture the city in 1529 proved a turning point that marked the end of Ottoman expansion into Europe. It remains to be seen—needless to say—whether the election of Alexander Van der Bellen, a septuagenarian center-left candidate, over Norbert Hofer of the far-right Freedom Party to Austria’s largely ceremonial position as president will prove quite as decisive a hinge in the continent’s history.

Yet the beleaguered supporters of an open, inclusive, and integrated Europe committed to continued alliance with the United States will take good news where they can. After the twin shocks of the British Brexit referendum to quit the European Union and Donald Trump’s upset victory, the constellation of European populist movements touting protectionist, nativist, and nationalist themes have reason to feel emboldened. These parties still face many obstacles, but they are benefiting from the same working-class alienation over demographic and economic change that lifted Trump. And now they can anticipate a like-minded ally in the White House, who may boost them either directly or indirectly.

Decisions like Austria’s choice of a president, or the 2017 French, Dutch, and German national elections, may seem obscure to most Americans amid the approaching fights over the Affordable Care Act or control of the Supreme Court. But the reverberations won’t be confined to Europe if voters there vest more power in conservative populist parties, which are committed in most cases to reconfiguring—and even unraveling—the economic and security arrangements that have bound Europeans to each other, and to the United States, for decades.

“What we are witnessing is a political threat to the post-World War II order created by Roosevelt, Truman, Churchill, and [German Chancellor Konrad] Adenauer to find a way to cooperate in Europe and trans-Atlantically,” said Ivo Daalder, the former permanent representative to NATO for President Obama.

In many European countries, the populist wave still hasn’t advanced much beyond the beach. Cas Mudde, a University of Georgia associate professor of international affairs who studies these movements, noted in a recent Foreign Affairs article that in European elections over the past five years, populist parties have averaged a relatively modest 16.5 percent of the vote. But conservative populist parties already control governments in Hungary and Poland and participate in governing coalitions in Finland and Norway. The populist party Alternative for Germany is establishing a foothold—though not yet more—there, and polls give Marine Le Pen, leader of the far-right National Front, strong odds of reaching the final round of next spring’s French presidential election.

Even the results of last weekend’s European elections sent a mixed message. On the same day that Austrians spurned Hofer, Italian voters rejected a constitutional rewrite backed by centrist Prime Minister Matteo Renzi. Though many groups and factors played a role, populists there contributed to the decisive defeat, which precipitated the quick fall of Renzi’s government and a resurgence of uncertainty about Italy’s political future (which is such a regular condition that it may qualify as its own form of certainty.) And though Hofer lost in Austria, he still attracted 46 percent of the vote—far more than might have seemed possible not long ago. “We are celebrating when [Hofer] didn’t win a majority, [but] 10 years ago we were upset when they had a third of the vote,” Mudde said in an interview, referring to conservative populist parties like Hofer’s. “We are now increasingly in a situation … where in many elections, the radical populist right is going to be the first or the second party and that is not going to change in the foreseeable future.”

Two big, new forces loom over this tense electoral struggle. One is Trump, who praised Brexit and responded with airy indifference (“It looks like it’s on its way”) when asked last summer if he feared the collapse of the European Union. Stephen Bannon, Trump’s incoming White House chief strategist, has praised the conservative populist parties as a “global tea party” movement and downplayed the xenophobia and racism that have infected all of them. Trump tweeted for Britain to appoint as its U.S. ambassador Nigel Farage, former leader of the nativist UK Independence Party, and recently told Time, without alarm, that he expected a continuing nationalist backlash across Europe. Trump’s administration could further legitimize these parties either by meeting with their leaders or simply by amplifying and validating their anti-globalist, anti-immigrant themes. “One of the arguments that the political establishment has used against the radical right is if you elect them you become a laughing stock and you become isolated [internationally],” Mudde noted in the interview. That case would be tough to sustain, he said, if Trump’s administration signals it will engage with provocateurs like Le Pen.

Equally important is how aggressively Vladimir Putin, emboldened by his success at roiling the U.S. presidential race, will covertly intervene to support these parties in coming elections; many take the recent WikiLeaks disclosure of internal German documents as a signal that Putin intends to hound German Chancellor Angela Merkel, the European leader now most committed to the Western alliance, as he did Hillary Clinton. “It is a divide-and-rule strategy,” said Daalder, now president of the Chicago Council on Global Affairs. “Putin is all about dividing Europe in as many different pieces as possible.” The new wild card in European politics is the previously unimaginable question of whether the next U.S. president finds that design dangerous, or compatible with his own disruptive vision of the continent’s future.

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