The US would not retaliate militarily if Russia used nuclear weapons against Ukraine. That is not part of US nuclear doctrine. Ukraine is not a treaty ally.
There are 6 things on the list. This one is the closest to promising to defend Ukraine:
>Seek immediate Security Council action to provide assistance to Belarus, Kazakhstan and Ukraine if they "should become a victim of an act of aggression or an object of a threat of aggression in which nuclear weapons are used".
I don't know exactly what "Seek immediate Security Council action" means. What if the US seeks it and doesn't find it? Is that in compliance with the memorandum?
While that is technically correct based on the UN Charter, it's meaningless in practice. None of the permanent Security Council members are willing to take direct action on this issue.
# 1 Respect Belarusian, Kazakh and Ukrainian independence and sovereignty in the existing borders.
#2 Refrain from the threat or the use of force against Belarus, Kazakhstan and Ukraine.
Both violated in the Krimean annexion 2014. Since Russia could easily violate this treaty, they could easily invade Ukraine now. And they got no weapons from signatories, only a few from the Baltics.
Treaties with these signatories are not worth the paper, otherwise Ukraine would have kept the nukes, and would have somehow got the codes also eventually.
The premise is not just that nukes are a deterrent against nuclear warfare, but also that nukes are a deterrent against conventional warfare. I'm not sure any country except israel is super explicit about this (cf Samson Option) but presumably the implicit or quietly explicit threat is there.
