KYIV — Turkey’s relationship with the United States is on the rocks, and the most immediate cause is Ankara’s refusal to get rid of its newly acquired, Russian-manufactured S-400 missiles that Washington says threaten NATO’s security. Yet when it comes to Ukraine, Ankara and Washington are in perfect lockstep vis-a-vis Moscow as witnessed by the outcome of Ukrainian President Volodymyr Zelensky’s April 10 meeting in Istanbul with Turkish President Recep Tayyip Erdogan.

A 20-point declaration signed by the two presidents left no doubt as to Turkey’s position. Notably, it called for an end to the “illegal and illegitimate annexation of Crimea, a former Ottoman dominion that is home to the Muslim Tatars, by the Russian Federation” and the “de-occupation” of the Donbas in Ukraine’s east, where Russian-backed Ukrainian separatists prevail. Turkey also reaffirmed its support for Ukraine’s membership in NATO and “its intention to obtain a Membership Action Plan in the near future.”

The show of support comes at a critical time. Over the past month, Russia has been massing troops, tanks and heavy artillery along its eastern border with Ukraine, the biggest build-up since its 2014 assault via Ukrainian proxies in the Donbas. The moves have set off alarm bells in Washington and NATO headquarters in Brussels, as Western leaders try to gauge Russian intentions. Is it about to mount a fresh attack and gobble up more chunks of Ukraine? Is it testing the resolve of the Joe Biden administration, which is taking a tougher stand on Russian misbehavior than Donald Trump did? Is it seeking to intimidate Zelensky, who since coming to power in 2019 in a landslide has been battling to beat back Russian influence, sanctioning pro-Moscow oligarchs and shuttering pro-Moscow news channels? Or is it a tactic to keep the conflict on the boil so as to ensure that Ukraine never joins NATO? The alliance’s rules stipulate that it does not grant membership to countries that are in conflict.

Few believe Russia is about to invade Ukraine, but the threat of an invasion provides leverage over Kyiv. Unlike the annexation of Crimea, a lush and balmy watering hole for freezing Russians, the absorption of war-ravaged Donbas would have little support inside Russia and would trigger a fresh round of debilitating Western sanctions. However, Western diplomats say military action by Moscow cannot be ruled out and needs to be taken seriously.