Leon Aron is director of Russian studies at the American Enterprise Institute.

Alexander Lukashenko, the dictator of Belarus, is facing the biggest threat of his 26-year reign. An extraordinary popular revolt has brought his regime to the brink of collapse — something that Vladimir Putin, mortally afraid of democratic contagion from a “color revolution” in his western neighbor, can hardly countenance.

Yet as startling as it might seem, the turbulence in Belarus also gives Russia’s president an opportunity — one he could seize with a high-stakes display of brazen military aggression that could go beyond merely cracking down in Belarus. Perhaps the most frightening scenario: an invasion of Lithuania. The Baltic republic, which shares a 420-mile border with Belarus, is a member of both the European Union and NATO.

Combining a raid on Lithuania with an intervention in Belarus could give Putin a chance to solve several problems at once. He could snuff out the threat of a Western-oriented regime in Minsk. He could drown his own people’s resentments in fear and patriotic exultation, as he did with the annexation of Crimea in 2014. And perhaps most important for him personally, he would revenge the fall of the Soviet Union by humiliating and wounding NATO — that predatory and perennially plotting bugaboo, which every Russian alive today has been taught to hate.

While hardly a cakewalk, such a scenario is well within the realm of the possible. The Russian and the Belarusian militaries have exercised together in Belarus for many years. Belarus’s top officers graduated from the same Soviet military schools and academies as their Russian counterparts. Just within the past few days, as Lukashenko’s fate appeared increasingly uncertain, the two countries have launched a new round of joint exercises, introducing a major element of uncertainty.

A raid would be a very limited operation. No tanks would roll into Vilnius. Seizing Lithuanian territory, perhaps a few kilometers deep on the Lithuanian side of the Belarus-Lithuania border to “thwart a NATO aggression,” and then showing that Russia had gotten away with it, would be the point.

Lithuania is host to a NATO battalion of 1,092 troops from Germany, Belgium and the Netherlands. Yet like the other two NATO battalions in Estonia and Latvia, the alliance troops in Lithuania are not there to stop a Russian invasion but to serve as a “tripwire,” an embodiment of Article Five of the NATO charter: An attack on one is an attack on all.

But tripwire deterrence is only as good as the tripper’s belief that it will trigger an explosion. And Putin may consider such an explosion less likely at a time when NATO disunity is probably at its greatest since the end of the Cold War. President Trump recently started withdrawing a third of the U.S. troops in Germany. German Chancellor Angela Merkel intends to boycott the Group of Seven summit in Washington, and the French president contends that NATO is experiencing “brain death.”

Moscow has already staged a rehearsal. The large-scale Zapad 2017 military exercise included a simulated “counteroffensive” against an attack from three fictitious countries bordering Russia and Belarus in the northwest. The war game included the test-firing of nuclear missiles and flights of strategic bombers over the Atlantic, Baltic and Norwegian Seas.

It is child’s play for the Russian troll factories to flood the Internet with disinformation — including “deepfake” images of a pending NATO invasion of Belarus from Lithuania. The Baltic republic has been among the most vociferous critics of Putin’s Russia, often harboring political exiles from Russia and Belarus — and it is currently hosting Belarus opposition leader Svetlana Tikhanovskaya.

Technically, Russia and Belarus have been a “union state” since 1999, so Putin would argue that military intervention to restore the “legitimate authority” in Minsk would not only be a “fraternal” Russian impulse but also a legal obligation.

When the Kremlin sent its troops into rebellious Czechoslovakia this month 52 years ago, Soviet citizens were told that the Red Army was moving to forestall an invasion by the West Germans. Putin can reasonably hope that the trajectory of the West’s reaction would resemble that which followed the crushing of the Prague Spring: fulminations, hand-wringing and sanctions. Yet Russia could probably cope as long as it can continue to sell its oil and gas — while betting on a willingness to “reset relations,” four or so years down the road with whoever is sitting in the White House.

In the meantime, the Kremlin’s geopolitical gains could be enormous. Like the U.S.S.R. before it, Russia would be permanently feared — or, as Putin sees it, respected. NATO’s Article Five, the cornerstone of collective defense, would be rendered a fiction. The alliance might start to unravel as countries on its eastern flank sought individual “accommodations” with Moscow. Ukraine’s hesitant drift to the West would be likely to be arrested for the foreseeable future as well.

It is just as obvious what needs to be done to forestall this dismal scenario: an emergency summit meeting of NATO, a quick vote to stand with Lithuania and, most of all, the deployment of troops as fast and as close to the Lithuanian border with Belarus as the complicated logistics allow.

But don’t hold your breath. It is hard to repair in days what has been neglected for years.

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