Dodo,

Oh!

They could really start to flow together only 14 years ago.

And I'm the one putting too much emphasis on nation-states...

I don't have the time for a debate on linguistic differentiation in the 14th century, the significance of the Zaporozhian Host in 16th century and cetera, but, dear Dodo, you are pulling a fast one there.

As the nationalist movements of the 19th century, you are indulging in anachronism and anti-causality. Gimme a Warp drive :> Those movements in Ukraine, but similarly in Germany, and pretty much everywhere in Europe, do not appear out of thin air. Following the French Revolution (France's last truly serious contribution to History), they formalise nationalities, making them explicit in contemporary terms but they do not create them.

As for the tensions between the Ukrainian majority and the Russian (or more largely Russian-speaking) minority, we're talking about two fairly recent phenomenons: the overlap between the 2 populations on the eastern marches of the modern Ukrainian territory (with some Ukrainians in Russia, by the way) and even more recently, the forced Russification policy under Stalin and the continued integration of Ukraine into the USSR after him.

By the way, regarding the Stalinian Russification, I must say I'm pretty impressed to see how a nation which is not supposed to exist has managed to preserve its identity under very serious pressure. I'm saying that as a Breton French, that is someone who knows quite a bit about enforced acculturation in modern times.