IN THE PINE WOODS AND PASTURES OUTSIDE VILNIUS, Lithuania’s capital, a corps of professionals moves through maneuvers as temperatures drop below freezing and a light snow begins to fall.

By weekday these men and women are attorneys, accountants, journalists, or film producers. One is the capital city’s mayor. On a weekend like this one in November, they don camo and combat boots, pull on lined parkas, and load assault rifles to practice target shooting and survival skills. Some will spend their Saturday learning to make a stove from a beer can or making pond water potable using a condom and a made-from-scratch filter. Others will crawl over damp, mowed hay amid explosions in simulated maneuvers against an imagined enemy.

Weekend exercises like these are part of “being ready to defend ourselves and help neighbors,” Vilnius Mayor Remigijus Šimašius told me by telephone, saying he often takes part in them. Across the Baltic region, citizen militias like these are ramping up readiness to counter what they believe is an imminent Russian threat.

Together with its neighbors Latvia and Estonia, Lithuania fears an invasion by Moscow like that which occurred in Ukraine—using the Baltics’ ethnic Russian population to foment dissent, then sending in military forces to protect them and ultimately occupy a country of 3 million.

Experts agree those fears are well-founded. Moscow in recent months has increased troops and moved in nuclear-capable ballistic missiles to Kaliningrad, a Russian enclave wedged between Lithuania and Poland. Russian airspace incursions over Baltic countries number in the hundreds since 2014, forcing NATO to double its air policing contingent.

Two years ago Šimašius helped revive the Lithuanian Riflemen’s Union, a regimented civilian corps founded in 1919 but banned in 1940 by the Soviets. When Russia invaded Ukraine in 2014, “It was a wake-up call,” said Šimašius. The 42-year-old politician joined with other young professionals and university students to begin weekend drilling, sometimes alongside Lithuania’s national guard. Today the Riflemen’s Union currently numbers in the thousands. A 2015 news photo showed the mayor drilling with a Riflemen’s unit, wearing camouflage and deploying a NATO-issue paint stick to smear his face green.

In a troubled world beset by myriad hot spots, few in the United States are thinking of Eastern Europe as the first flash point for the Trump administration. But perhaps the most urgent foreign policy threat confronting President-elect Donald Trump will be the future and stability of the NATO alliance—a pact Trump called into question during his campaign.

“NATO is the most important international institution in the world, the linchpin of liberal order,” said Paul Miller, associate director of the Clements Center for National Security at The University of Texas at Austin. “If the United States walks away from NATO, or if NATO degenerates into a meaningless talk-shop with no real cooperative security to bind it together, the geopolitical clock will have been rewound to 1939—and the world is the most dangerous it’s been since World War II.”

President-elect Trump jockeyed in December to put together a foreign policy team and to begin to establish top-level ties, sometimes via controversial courtesy calls, with heads of state. Unlike his predecessor, who entered the White House with an American public decidedly pulling away from international conflicts, Trump is under close scrutiny over how he will handle himself on the world stage. Chief among the concerns: Will a President Donald Trump try to build a relationship with Russian President Vladimir Putin at the expense of the Baltics and NATO?

During the election campaign, Trump declared himself ready to work with Putin and said he might consider recognizing Crimea, the Russian-annexed portion of Ukraine, and lifting sanctions against Moscow. He also called into question the NATO treaty obligation making an attack on one of the 28 member states an attack on all.

Asked about NATO in Cleveland during the Republican convention in July, Trump complained many NATO nations “are not making payments.” Could NATO members expect the United States to come to their military aid if they were attacked by Russia? asked reporters from The New York Times. Trump responded: “Have they fulfilled their obligations to us? If they fulfill their obligations to us, the answer is yes.”

Whether Trump will submit to NATO deployments could be tested early on: Responding to Moscow’s continuing show of force, NATO heads of state in July agreed to move troops to positions along Russia’s border in 2017. The multinational battalions will be led by Germany in Lithuania, by Canada in Latvia, by British commanders in Estonia, and by the United States in Poland.

Russia could act in some way to counter the defensive buildup. Miller said he expects likely flash points to be the three Baltic nations, though Romania and Poland are also possibilities. Estonia, Latvia, and Lithuania are important, said Miller, because they are the only NATO member states that were formerly republics of the Soviet Union.

According to Miller, “The United States should treat them with the same importance as France, Germany and the United Kingdom. If the NATO Article 5 guarantee isn’t enforced for the Baltics, it won’t be enforced anywhere, and NATO is dead. The Baltics are the test case for the solidity of the security guarantee.”

Article 5 enshrines the collective defense requirement at the heart of the treaty. NATO has only invoked it one time: On the evening of Sept. 12, 2001, less than 24 hours after Islamic terrorists attacked the United States on 9/11.

TWELVE HEADS OF STATE, including President Harry Truman, signed the pact creating the North Atlantic Treaty Organization in 1949. More than 36 million Europeans had died in World War II (more than half of them civilians), and the continent was awash in refugees and orphans. Half a million people were homeless in Hamburg, Germany, alone.

Beyond resisting Soviet aggression, the pact’s original signers hoped to stave off the kind of militant nationalism that gave rise to Hitler’s Germany. For the United States and Canada, NATO participation represented an acknowledgment that the Atlantic Ocean was not bulwark enough against global conflict. As NATO’s first secretary general, Lord Ismay, put it, the strategic objective was “to keep the Russians out, the Americans in and the Germans down.”

With the fall of the Berlin Wall and the breakup of the Soviet Union, NATO grew to encompass 28 nations, including 10 former members of the Warsaw Pact, the communist alliance that faced off against NATO in the Cold War years. With expanded membership, it reset the alliance’s mission. At the end of the Cold War, NATO agreed, the world faced “complex new risks to Euro-Atlantic peace and security, including oppression, ethnic conflict, economic distress, the collapse of political order, and the proliferation of weapons of mass destruction.”

The terrorist attacks of 9/11 proved that mission’s foresightedness, and NATO members led intervention in Afghanistan to deny al-Qaeda a base of operations.

Critics, including some Republicans, have agreed with Trump that NATO partners should increase their defense spending toward the alliance. Military expenditures as a percentage of a country’s economic output vary widely across the 28 members of NATO, which includes some of the oldest armies in the world but also some of the newest—like Slovenia, which became a country in 1991 and a member of NATO in 2004.

The NATO guideline calls for members to spend an average of 2 percent of gross domestic product on defense. While the United States spends nearly 3.6 percent of GDP, Germany spends only 1.2 percent of its GDP on defense. But since 2014, military expenditures by European members and Canada have increased by 3 percent, or $8 billion, according to NATO Secretary General Jens Stoltenberg.

MUCH OF THE U.S. STANCE TOWARD NATO will depend on Trump’s top defense and foreign policy picks. Retired Marine Corps Gen. James Mattis, named in early December to be secretary of defense, has differed with Trump over NATO, criticizing him during the campaign and warning that Russia’s intentions are “to break NATO apart.”

“Some of those allies have lost more troops per capita in Afghanistan than we have,” said Mattis, who served as commander of U.S. Central Command over Afghanistan from 2010 to 2013. “Some of them are spending 20 percent of their national budget on defense.”

Among political leaders, the rising tide of populism and nationalism—in Europe and the United States—could play a determining factor in their stance toward Russia and their support for NATO (see below).

For Putin and other Russians who see the world through the lens of Russian religious nationalism, said Miller, “the West is inherently a threat because of its degeneracy and globalism. In this view, NATO is not the benign guarantor of liberal order in Europe, but the hostile agent of the degenerate West and the primary obstacle to Russian greatness. Thus, Putin’s grand strategy requires breaking NATO.”

On the front lines in Vilnius, that helps to explain why Mayor Šimašius and others are taking their own readiness more seriously. “It is a realistic approach. If you want to have peace, you have to be ready to defend it. That’s the understanding,” he said. “Before we were in one big family and that is enough. Now we understand it is not enough.”

An era in flames: Europe’s changing political landscape

Russian President Vladimir Putin’s “religious nationalism” and his blatant efforts to recapture a Cold War–era sphere of influence come at a time of drastic—and potentially destabilizing—change across Europe.

Anti-EU populist movements are sweeping the continent. Carried along by a wave of anti-establishment discontent over high unemployment, refugee migration, and politics as usual, the transformation is highlighted by recent elections and Britain’s summer decision to leave the EU. What’s less clear: whether such breakups will lead to a security breakdown when confronting Putin’s threats.

Italy: Prime Minister Matteo Renzi on Dec. 4 announced his intent to resign after losing a constitutional referendum by 20 percentage points, setting the stage for the growing populist Five Star Movement and its founder, comedian and actor Beppe Grillo: “An era is going up in flames,” Grillo said after watching Donald Trump’s victory. “It’s the risk-takers, the stubborn, the barbarians who will carry the world forward.”

Austria: Populist far-right candidate Norbert Hofer suffered defeat in a Dec. 4 runoff presidential election. But Hofer, who had pledged to take Austria out of the EU, lost to independent (and former Green Party leader) Alexander Van der Bellen only after an appeal from a Holocaust survivor to vote against him went viral.

Spain: A 10-month-old caretaker government ended in October with the swearing in of the conservative People’s Party. But political deadlock has only increased the popularity of Podemos, the leftist populist party of former university researcher and lecturer Pablo Iglesias. While calling Trump “a fascist,” Iglesias hasn’t run from comparisons to Trump’s success, saying, “Populists are outsiders, they may be from the right, the left, ultra-liberal or protectionists.”

France: National Front Party leader Marine Le Pen called Trump’s win “a sign of hope” showing “people are taking their future back.” But France’s Republican Party may have upstaged Le Pen in nominating François Fillon, a socially conservative former prime minister and an adherent of Margaret Thatcher’s economics, to run against her in elections next year.

Britain: Foreign Secretary Boris Johnson draws comparisons to Donald Trump for looks and personality, and faces the same media onslaught, but the “one-nation” conservative was brought in by Prime Minister Theresa May and is proving both adept and assertive on foreign policy. “We can’t normalize relations with Russia or go back to business as usual,” he said in a Dec. 2 speech.

Netherlands: Long before the current crop of populist leaders, Geert Wilders staked a reputation on opposing the Islamization of Europe, but the Party for Freedom founder and leader is on trial for hate speech and lost some of his mainstream conservative supporters when he asked supporters if they wanted “fewer or more Moroccans” in the Netherlands. When they chanted back “fewer,” Wilders replied, “We’ll organize that.”

Germany: The grand dame of European politics, Chancellor Angela Merkel, is up for reelection to a fourth term, but faces growing opposition from Frauke Petry of the Alternative for Germany party over her “open-door” policy toward Middle East and African migrants. A serious challenger has yet to emerge, but Merkel could face her toughest election yet: It's scheduled for September 2017.