What treaty is that? If you mean the Budapest Memorandum of 1994, it’s not a treaty ratified by Congress, and besides, it does not state we are obligated to use nukes. Have you read it?
Sorry it’s WaPo, but this explains it:
In any case, that Budapest memorandum ended when the Ukrainian government ended in a western funded nazi extremist coup.
If you have an agreement with your next door neighbor about who trims the hedge between your properties, and someone breaks in and murders him. You are not bound by the agreements with you dead neighbor.
The Galician nazis had a violent coup, and started shelling ethic Russians in the east who opted out of their new junta in Kiev.