What if the deadbeat liar breaks a previous promise to steal some of your stuff, spends the next few years accusing you of starting hostilities, waits for all your friends to look uninterested then turns up with his henchmen on the doorstep and says "honestly, all we want is your friends to promise never to help you?"

The extreme stakes make it far more likely that he wants the expansion he made the declaration of war speech about (and even more to crushing the idea of a potential successful large democracy full of Russian speakers), not an empty promise concerning something that wasn't likely to happen anyway and is of little interest except in ensuring ease of invading Ukraine. The Ukraine border is of zero strategic value to NATO since they're not remotely interested in or capable of mounting a ground invasion against a nuclear power whose terrain is where world conquerors go to die (and pretty much the only conceivable circumstance where they might have done it is in response to Russia invading Ukraine... the thing he actually did). They don't exactly have happy memories of fighting in Finland either, which is where their border with a newly galvanised NATO will end up instead.

It's even more transparently bullshit version of the preemptive defence doctrine than WMDs in Iraq, to the extent he had to manufacture a secondary excuse for domestic consumption because even a public that buys the idea he's shelling Kiev to defending the Donbas from atrocities won't believe he had to do it to save the country from NATO invasion.