Actually just looked into this. Budapest Memorandum did not include any assurance of defense. Merely that no one on either side would leverage nukes against Ukraine. Though the Russians are in flagrant violation of section 2 thereof, namely

>The Russian Federation, the United Kingdom of Great Britain and Northern Ireland and the United States of America reaffirm their obligation to refrain from the threat or use of force against the territorial integrity or political independence of Ukraine, and that none of their weapons will ever be used against Ukraine except in self-defence or otherwise in accordance with the Charter of the United Nations.