KYIV, Ukraine—In December, the local edition of lifestyle magazine Elle announced a new fashion on its cover: It would publish only in Ukrainian, not Russian.

The switch in languages for a publication that is hardly a hotbed of nationalist ferment goes some way toward explaining why Russian President Vladimir Putin is forcing the issue of Ukraine now. Since Russia first invaded portions of the country in 2014, Ukraine has been severing cultural ties with its old imperial ruler and developing a keener sense of what it means to be Ukrainian. That undermines the argument Mr. Putin routinely cites as justification for his interventions here—that Ukrainians are essentially Russians.