I don't think your reading lines up with history. Putin was clearly content to allow Ukraine a large degree of sovereignty and autonomy as long as they remained under an authoritarian system. If nationalism was the only driver, why overthrow Yanukovych, thereby spitting in Putin's face and risking domination by another country?
What wasn't tolerable to Putin was a bourgeoning democracy on Russia's doorstep with similar ethnography and demography to Russia. If it proved to be more successful than Russia's model (which isn't hard), that's a clear threat to his regime.
'I think this has extraordinarily little to do with any "political system", of which Ukraine and Russia were both quite similar.'
It seems like you're simply ignoring what happened in 2014. The stark difference between Ukraine and Russia's political systems (and their future trajectories) after that point is one of the main causes of the war.
The "authoritarian system" in question was russia's ethnic-imperial system of empire. That it was "authoritarian" is far less important than its being Russian, ethnically and culturally.
The offense to russia was first to turn to the west, and hence as Russia sees it, a counter-empire; and the secondly, the suprise and outrage, to believe that it's a Nation.
Both are incomprehensible to Russia -- it has nothing to do with how "authoritarian" anything is.
I see some truth to your point, but it's also very reductive, and you're providing nothing to back up your reductionism.
The "turn to the west" is geopolitical but it's also a turn away from a conservative authoritarian order and toward a liberal democratic order. Is that just a meaningless geopolitical coincidence? No, it's clearly part of the equation, though certainly not the only part.
Well my point is only that a person reading my comments comes to see their "intuitive nationalism" as an explicit feature of their thinking, rather than a natural fact of the world.
My analysis doesnt need to be 100% to show that even the very idea of "invasion" in the modern sense is full of contingencies we don't acknowledge.
What a weird thing, no, in the history of the world that the US invades iraq and wishes for it to govern "itself".
Once you remove the "Nation" from your thinking, various issues become clearer, esp. why so many "countries" appear unstable. Ie., politically they are countries, but havent yet "progressed" to "default nationalism".
Once a region adopts nationalism, it seems there's no going back; and people of that Nation are fundamentally radicalised by that notion. There are "borders", "immigration" and indeed -- how strangely -- "illegal" immigration; there are armies, and you should join one if you're "invaded".
These ideas appear in our thinking as transparent, obvious, facts of the world; and if we feel they are violated, then we feel outraged -- and would act very severely to get redress. This is radicalism, and a certain "liberal nationalism" has deeply radicalised the west.
I think, foremost, we want Ukraine to fight Russia because we believe Ukraine to be a Nation. I think something many of its own people did not think 20 years ago, and now, many die because they believe it.
I actually like the point you tried to develop in this thread, because I feel most people don't question their assumptions often enough. However, you are clearly reducing complex phenomena to a single cause, incorrectly so in my opinion.
Greek city-states fought each other all the time. They definitely had a concept of "Nation", even though they would have a different name for it. All patterns you associate with Nationalism were there, including xenophobia - that's where the goddamn word was created. The rise of what we call Nationalism in modernity has its own peculiarities, including the rise of military-industrial complexes that have every incentive to weaponize it as well. But it is simply wrong to think that, for example, the issue between Ukraine and Russia can be meaningfully reduced to one or the other side "adopting nationalism". Also, it is factually incorrect to say 20 years ago Ukrainians (or inhabitants of that region) would have wanted to be ruled by Putin or Russia in general.
What wasn't tolerable to Putin was a bourgeoning democracy on Russia's doorstep with similar ethnography and demography to Russia. If it proved to be more successful than Russia's model (which isn't hard), that's a clear threat to his regime.
'I think this has extraordinarily little to do with any "political system", of which Ukraine and Russia were both quite similar.'
It seems like you're simply ignoring what happened in 2014. The stark difference between Ukraine and Russia's political systems (and their future trajectories) after that point is one of the main causes of the war.