As Russian forces dig in for a possible offensive, NATO’s frenzy to arm Ukraine zeroes in on Moscow’s Black Sea pressure point.

(Originally published Jan. 19 in “What in the World“) The U.S. is now reportedly considering giving Ukraine whatever weaponry it needs to launch a full-scale assault on Russian-occupied Crimea.

Washington has held back for fear of prompting an escalation of the war by Russian President Vladimir Putin, including a direct assault on members of the North Atlantic Treaty Organization, use of tactical nuclear weapons in Ukraine, or worse, the launch of nuclear weapons against NATO.

But the West has become increasingly bold about escalating military aid to Ukraine’s efforts to repel Russia’s invasion. In May, U.S. President Joe Biden caved on allowing shipments of M177 howitzers to Ukraine. By the end of the month, Biden had also reversed his refusal to supply Ukraine with M142 High Mobility Artillery Rocket System, or Himars, a reversal so significant that Biden took the trouble of explaining his decision to The Times. Last month, Biden promised Ukraine a Patriot missile battery after Ukraine showed that, even without long-range American weapons, it had the capability to organize attacks inside Russia.

There are competing explanations for this new willingness to risk antagonizing Putin. Some say it’s to break the ferocious stalemate the war has settled back into, with Russian forces hunkering down in defensive positions, and thus prevent the war from dragging on for years, allowing Russia to cement its control over Ukraine’s southeast. Others say it’s in response to Ukraine’s increasing warnings of a new Russian offensive, as Russia plans to mobilize new conscripts, its air force (still largely MIA in the war thus far) conducts joint exercises in Belarus, and the Russian Black Sea fleet mobilizes ships and submarines out of Novorossiysk (another Russian Black Sea, warm-water port that somehow never seems mentioned in discussions of Sevastopol’s exclusive importance to Moscow because of its smaller size and shallower harbor). But the most likely explanation for the West’s newfound boldness is the increasing signs of Moscow’s weakness. The U.S. and NATO simply confuse Russia’s plight with a decreasingly likelihood that Putin can or will escalate.

Whatever the catalyst, U.S. and NATO members are rushing to supply Ukraine with armored personnel vehicles and, as of last week, battle tanks ahead of what Ukraine has warned may be a new Russian offensive. With Germany now saying it won’t allow Denmark, Finland or Poland to ship their own German-made Leopard tanks to Ukraine unless the U.S. sends tanks of its own, the Biden Administration is preparing a new, $2.5 billion weapons package for Ukraine that includes more Bradley fighting vehicles and, for the first time, Stryker armored combat vehicles.

Still no tanks, but the Pentagon has argued that the M1 Abrams is too complex and too much of a gas guzzler to be useful in Ukraine. Other than the Abrams, the only remaining weapons Ukrainian President Volodymyr Zelensky has still been denied are attack drones, fighter jets and long-range missiles.

That steady escalation represents a rapidly shifting strategy: from giving Ukraine just enough to avoid defeat and trap Russia in a crippling quagmire, to one of providing Ukraine with enough weaponry to drive the Russians out and defeat Putin. Since Zelensky has rejected ceding any territory to Russia, Crimea is essential to that victory. And to take Crimea, Ukraine first would need to cut the land bridge to it that Russian forces have created across southeastern Ukraine.

But Crimea may be the very chess piece Putin is willing to defend at all costs, including nuclear war. U.S. intelligence has said that Putin sees victory in Ukraine as vital to Russia’s very survival. There is a cultural-historical element to this—Ukraine as home to the original Kievan Rus. And there’s a historical military element in that Ukraine (and Poland) is the football pitch across which Europe’s invading armies have repeatedly played out ambitions to invade Russia.

The clearest source of Russian interest in Ukraine, however, is Crimea, specifically the port of Sevastopol. As any history student will know, Sevastopol was always vital to Russia and the Soviet Union because it is its only warm water port (except for Novorossiysk, anyway). When it lost Sevastopol when Ukraine became independent in 1991, tensions arose immediately, with some in Russia’s parliament agitating for annexation of Crimea. In 1997 a treaty was signed that allowed Russia to lease part of the naval base at Sevastopol.

Russia’s claims to Crimea, as in Donbas, are buttressed by the fact that most residents there identify Russian as their first language. It was protests in 2014 by Crimean residents against the ouster of Ukraine’s pro-Russian president Viktor Yanukovych that served as a pretext for Russia’s annexation. Concerns that Ukraine may have been edging towards membership in NATO, some argue, precipitated Russia’s invasion a year ago.

Just how relevant having an ice-free port on the Black Sea is in modern military warfare is debatable. But its importance to Moscow can’t be understated. Some pundits have argued that the war can only be ended by somehow sating Russia’s historical lust for Sevastopol. And others, including the head of the U.S. Joint Chiefs of Staff Gen. Mark Milley, have said Ukraine cannot defeat Russia conventionally without some kind of negotiated settlement.

As the war drags on and the casualties mount, Ukraine may start running out of more than ammunition. Given the U.S. aversion to walking away from sunk costs, it seems only a matter of time before Washington has to weigh shipping an even more vital strategic weapon to Ukraine: U.S. troops.