When President Volodymyr Zelensky was slated to open the annual Yalta European Strategy conference in Kyiv last month, he yielded the honor to Ukraine’s new national hero, filmmaker Oleh Sentsov. Just released from five years in Russian captivity as part of a prisoner swap, Mr. Sentsov brought the audience to tears as he recounted how he had struggled with his jailers to hold on to a prized possession: a plastic jar decorated with Ukraine’s blue-and-yellow colors. He ended by holding up the jar, to thunderous applause.

Mr. Sentsov narrated this tale in Russian. Like many other ethnic Russians from Ukraine’s Crimea region, Mr. Sentsov—imprisoned for opposing Russia’s forced annexation of the peninsula—isn’t fluent in the Ukrainian language.

Mr. Zelensky, born to a Russian-speaking Jewish family, noted in his own speech a few minutes later that he, too, had to learn to speak Ukrainian in adulthood. That background didn’t prevent him from getting elected in the largest landslide in Ukrainian history, winning 73% of the vote last April.

Under the old Soviet Union’s concept of nationalities, which defined Ukrainian-ness by blood rather than by mind-set (and recorded ethnicity on identity cards), neither man would have even qualified as a Ukrainian. In its 28 years of independence, however, Ukraine has moved so decisively in embracing an inclusive, almost American, model of nationhood that the very concept of an “ethnic Ukrainian” has become obsolete in mainstream political life.

That process has accelerated in recent years—as highlighted by Mr. Zelensky’s triumph—even though the country has been bruised by a war with Russia and a resulting economic crisis, the kind of circumstances that have ignited ethnic hatreds elsewhere. Ukraine’s trajectory stands in contrast to many wealthy Western democracies where “blood and soil” nationalism and anti-immigrant feelings have surged in the same period as a potent new force. Instead of splitting Ukraine along ethnic lines, the loss of Crimea and the ongoing conflict in the eastern region of Donbas—where Russia has fostered separatist statelets in the name of protecting Russian-speakers’ rights since 2014—have had the opposite effect of strengthening unity in the rest of the country.