Is the United States obligated by treaty to come to Ukraine’s defense? No?
***Uhh, yes. We signed a sovereignty assurance agreement, they honored it, the Russians violated it, and we’re abrogating it.
https://en.wikipedia.org/wiki/Budapest_Memorandum_on_Security_Assurances
From your link:
"[The US] ...agreed to the following:"
"Seek immediate Security Council action to provide assistance to Belarus, Kazakhstan or 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'."
It would appear that the US is obligated to seek UN action - NOT intervene militarily on a unilateral basis, as so many are advocating...