Narva, a small Estonian town that borders Russia, used to be part of the USSR. Most people there speak only Russian. Will Putin try to invade like he did in Crimea?

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.”

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.

Without commenting on any alleged relationship between Trump and Putin, or on the injustice of Russia’s meddling in the internal affairs of its neighbors, it is starting to look like expanding NATO to include not only former members of the Warsaw Pact but also former members of the Soviet Union itself (contrary to promises!) was a foolish idea. This was spiking the football, followed by excessive end zone celebration – a 1990s-era End-of-History neocon victory lap. It has now put NATO in a potentially very awkward position. This quite apart from the liability of taking on people who were until very recently on the other side, and who might retain some of their old sympathies.