I don't really understand why Putin has "gone for it" now, particularly. It doesn't make a lot of sense to me. Anyone not a fully paid-up member of the Trump cult surely can see that, if anything, Biden had a much greater probability of producing a coherent response (certainly, of bringing allies with him) than Trump would have done. The opinion poll results even now apparently show more Democrats favouring a military reaction than Republicans (though personally I'm slightly relieved that it's a minority of both groups - talk here about the RAF "providing air support" for the Ukrainians makes me nervous as to where that could lead).
I mean, maybe, one could stretch to the possibility that Trump's irrational unpredictability might have been a kind of deterrence in itself. The 'madman theory' of deterrence. Or maybe it's just that with Trump in charge of the US Putin felt he was winning anyway so could bide his time? Now he's suddenly in a hurry (to go where, though? to accomplishy what, exactly?)
But honestly I did not expect Putin's military posturing to come to anything. It's fortunate I'm not in charge of anything, because I got this one 100% wrong - I really was sure it would all blow over after a bit of (Cossack) sabre-rattling. I do not understand why Putin has done this, particularly right at this moment.
It seems quite mad on Putin's part (their track record in Afghanistan and Chechnya surely can't inspire confidence on the Russian side?). So that 'madman theory' feels like it now applies the other way. I'm worried he's become irrational. I mean a lot of leaders, even in functioning democracies, go a bit peculiar after being in power too long. It was bad enough having Trump in control of a nuclear arsenal, but at least the US had some semblance of democracy and 'checks and balances'.