No, it isn't. Make that a condition of membership. Absolutely zero reason that couldn't work. And if we look like we're breaking that pledge, that's when you may have an argument to invade.
Do you really think the U.S. government is in any way trustworthy? Do you think they would ever make such an agreement given their extreme bellicosity and aggression? You must have a much better impression of Bidenistan than I do.
In any case the war is on. Best case is quick Russian defeat with a way to save face so the tactical nukes don’t fly i.e. they get the Donbas and Black Sea areas they want. It’s better that the Ukraine gets chopped up and steamrolled than nuclear war. A million dead Ukies over thermonuclear holocaust any day of the week. I’m not pro-Russia and I’m not pro-Ukraine I’m pro-personal survival and prosperity.