I'm beginning to form a hypothesis about why Putin's "Russians and Ukrainians are one people" rhetoric is such a big and important part of the background to this invasion: This whole concept of Russia growing out of Kievan Rus has, as I understand it, long been part of the Russian self-image. In reality, though, at least as far as the actual Russian state is concerned, that's not true at all -- Russia is the descendant of Muscovy[1], not Kiev (=Kyiv). But, as that contrasting-pictures-of-cathedrals-vs-forest meme that did the rounds last week so brilliantly illustrates, Muscovy has pretty much no early history at all -- AFAICR it really rose to prominence only after the Mongol invasions, so it's late-medieval. Kiev, OTOH, goes back to at least the Vikings and Byzantines, so pre- or early-medieval.

So I'm beginning to think this whole conflation of Russia's history with Ukraine's is just Russia looking to shore up its histirical legitimacy. Rulers, particularly autocratic and authoritarian ones, have always wanted to make their country look ancient and venerable: The Swedish Vasa kings consigned a Historia from Olaus Rudbeck that purported to show how Sweden was actually Atlantis reincarnated; Czar Ivan The Terrible, from the original to the current incarnation, tried to usurp Ukraine's history as that of Muscovy.

(No idea how original this is; probably not at all. I mean, it's hard to believe that real historians would have missed it all these years... But not having studied anywhere near enough history, I hadn't seen it before I came up with it on my own.)

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[1]: And, OK, I suppose to some extent of Novgorod.