In order to succeed, imperialists typically have to believe that they represent a higher order of civilization than the people they aspire to rule. For the imperial regime to last, the colonized people have to believe it, too. Once that belief is gone, the empire ultimately goes with it.
Putin’s invasion of Ukraine is a large part of an imperial project. Since he insists that Ukraine is an integral part of Russia–in essence, Russia’s little brother, to be bossed around by big brother–he can’t very well argue that he is bringing civilization to the great unwashed. How, then, does he justify his claims?
I think, from Putin’s perspective, the analogy he would find most appropriate is Lincoln waging war on the Confederacy to maintain the Union. He’s wrong, of course; the historical relationship between Russia and Ukraine is far longer, and more complex, than that. But what else could he say?