NARVA, Estonia—If you haven’t heard of Narva, you might very soon. This small, mostly Russian-speaking city lies along Estonia’s boundary with Russia, separated geographically from its larger neighbor only by a partially frozen river. A 13th-century castle towers over passersby, while an intimidating medieval stronghold stares back across the river from the Russian side. A short walk away stands a monument to the late chess grand master Paul Keres, who was born here and lived through decades of Soviet occupation, but always attributed his success to the Estonian school of chess.

This city is also the epicenter of what could be an epic challenge for Western military alliances—what NATO calls the “Narva scenario”—one that would test the foundation underpinning the security partnership.

When Estonia regained its independence after the Soviet Union collapsed in 1991, Narva became a border town. Street signs here are in Estonian script, official business is carried out in Estonian, and the country requires that anyone who becomes a citizen must speak the language. But Narva’s population remains overwhelmingly Russian-speaking and ethnically Russian, leaving a sizeable number ineligible for Estonian citizenship. Instead, many are either Russian citizens or stateless residents of Estonia, who possess gray “alien passports.”

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All of that, Western security officials fear, makes it a prime target for Russia and its leader, Vladimir Putin. Under the “Narva scenario,” NATO worries that Putin could try to claw Narva into Russia. Such a move would mimic Russia’s incursion into Crimea, a Ukrainian territory it annexed in 2014, and its efforts to sow unrest in eastern Ukraine. But a similar push into Estonia would have even farther-reaching consequences: Estonia is a member of NATO.

Such a situation would then be a test of NATO’s commitment to Article V of the Washington Treaty, the one-for-all, all-for-one provision that requires the 29 NATO members, including the United States, to come to the defense of other member states.

Article V has only been invoked once, in the aftermath of the September 11 attacks on the United States. If it were used again, this time at the request of Estonia in response to Russian aggression, would the entire alliance—crucially, the United States under President Donald Trump—come to Estonia’s aid? The New York Times reported this month that Trump has considered withdrawing from NATO, and the president has questioned the alliance’s efficacy publicly on several occasions. Remarks and reports like these are once again raising questions about his commitment to countries like Estonia, and sparking fears that he is emboldening Putin.

In 2014, when Russia invaded eastern Ukraine and swallowed Crimea, Putin painted himself as the protector of Ukraine’s Russian-speaking minority. “Millions of Russians went to bed in one country and woke up abroad,” Putin said, referring to the fall of the Soviet Union. “Overnight, they were minorities in the former Soviet republics.” Kremlin-controlled media claimed that the new government in Kiev posed an imminent threat to the Russian-speaking population in eastern Ukraine. After the invasion, Crimea held a (widely discredited) referendum in which 95 percent of the population purportedly voted to become part of Russia. Across eastern Ukraine, Russian-backed separatists continue to wreak havoc and undermine Ukraine’s sovereignty.

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Estonian defense officials I met this month in Tallinn, the capital, bristled at the notion that Narva could be the next Crimea. Estonia is not just a member of the European Union and NATO, they say; it is also far less corrupt than Ukraine, and while the average income in Narva is the lowest in Estonia, it is still almost double the level across the river, in Ivangorod, Russia.

But even as Estonian government officials dismiss concerns about Narva’s future, they are racing to further incorporate Narva into the rest of Estonia. A nationwide survey last year found that while almost 90 percent of Estonian-speaking respondents favored NATO membership, just 32 percent of Russian speakers backed Estonia being part of the alliance. The last thing the Estonian government wants is for Putin to claim to be the defender of Russian speakers in Narva, as he did in Crimea. For one month last year, Estonia’s president, Kersti Kaljulaid, took the unprecedented step of moving her office from Tallinn to Narva. To counter the influence of Russian-language media, Estonia is also pouring in development funds and using arts and culture to try to improve life here.

Estonian officials project confidence that if Russia decided to invade, NATO allies—and particularly the United States—would come to their defense. They cite the mutual-defense provision of the Washington Treaty, and add that Estonia has not only deployed troops to Afghanistan, but also been one of the few nations to spend 2 percent of its GDP on defense, a Trump hobbyhorse. Lauri Kuusing, the foreign-policy adviser to the Estonian president, notes that Kaljulaid visited Washington last year for an Oval Office meeting with Trump, and Estonia has regular contacts with the Pentagon, the State Department, the vice president’s office, and Congress.

But Kalev Stoicescu, who was Estonia’s ambassador to Washington from 1997 to 2000, was not so sanguine. Trump “worries everybody” in Estonia, he told me. Trump called NATO “obsolete” during his campaign and reportedly “threw a tantrum” during a NATO heads-of-state meeting in July. At a G7 summit last year, Trump apparently told fellow world leaders that Crimea should be part of Russia because most people there speak Russian.

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Congress has sought to limit Trump’s damage to the alliance, including by overwhelmingly passing a bipartisan resolution affirming U.S. commitment to NATO last year. Legislators could also seek to prohibit the president from unilaterally withdrawing, as a bipartisan group of senators proposed last week. The House of Representatives passed such a measure by a 357–22 vote this week, and while the Senate has not yet taken it up, the new head of the Senate Foreign Relations Committee has said that there is no appetite in the upper chamber for pulling the United States out.

But ultimately, whether Trump moves to formally withdraw from the alliance or not, his skepticism toward NATO could have an immediate effect on borders and lives in parts of Europe like Narva. The president’s words and tweets can still inflict grave damage on the alliance. Uncertainty about Trump’s commitments to NATO makes a military miscalculation more likely, not less.

If Putin assesses that under Trump, the United States will not follow through on its treaty commitments to allies in Europe, NATO—and with it, America’s word—could turn into a house of cards, and Narva might be the first to fall.