This is definitely about humiliation of the West and subjugation of Ukraine.
Putin doesn't care about defensible border with China. In fact they don't care about the Chinese ambitions in South Siberia at all. And those ambitions are much clearer than any idea that NATO or European countries might want to invade Russia.
Putin's only worry is avoiding the fate of Sadam and Qaddafi and for that, in his mind, he needs to keep waging wars around Russia (and support struggling dictators all over the world, but that's unrelated to this conflict).
Exactly, if Putin really worried about protecting borders, he'd never move most of his Asian army units to the Ukrainian border, exposing huge vulnerability to the powerful autocratic nuclear regime who actually kinda wants these exposed lands.
The fact that he did it means that either he really believes that NATO is scheming on invading Russia proper, in which case he's just gone mad, or all this pseudo-historical pretext is just a bunch of crap he feeds the world to legitimize his personal quest for glory and give his worldwide supporters ("useful idiots" as they call them in KGB) a reason to let him off the hook for that.
They don't care about a defensible border with China because they are allied with China currently. If that changes you'll start seeing militarization of that border also.
Putin doesn't care about defensible border with China. In fact they don't care about the Chinese ambitions in South Siberia at all. And those ambitions are much clearer than any idea that NATO or European countries might want to invade Russia.
Putin's only worry is avoiding the fate of Sadam and Qaddafi and for that, in his mind, he needs to keep waging wars around Russia (and support struggling dictators all over the world, but that's unrelated to this conflict).