... and it's Finland, 1939-1940.
Russia hasn't had stellar success of late, even in Georgia and Chechnya, both a damn' sight easier to chew off than Ukraine.
But the real pain must come whenever they think of Finland, whose heavily outnumbered forces dealt them a serious bloody nose. It's generally reckoned the Soviet casualties in the 'Winter War' were of the same order of magnitude as Britain suffered during the whole of WW2 - and that in just three and a half months. For not much gain (a bit of Donbass-style land-grab), and substantial international opprobrium. (Not that Joe Stalin cared very much. Nor about casualties, for that matter.)
That's what can happen when you take on a nation that is (a) very big, geographically, with unlimited ability for tactical retreat by the defender, and overstretch trouble for the attacker; (b) fiercely nationalistic. Given their own dealings with Napoleon and Hitler, we might imagine that for the Russians, this lesson is not lost ...
It's been fairly noted recently that successful guerilla wars are fought by youthful fanatics, not middle-aged populations - with the suggestion that under this precept Ukraine has the wrong demographics for that kind of resistance. Maybe so: but surely Finland is a much more relevant case study.
Oh, and that extremely interested observer President Xi will also be recalling China's own less-than-glorious attack on Vietnam in 1979. Still, at least Putin hasn't pooped on Xi's Winter Olympics party in the unseemly way he did with Georgia, slap bang in the middle of Beijing's 2008 summer games, to China's profound disgust. Well - not yet, anyway.