Defense may be too strong a word. The signers seem to have interpreted "defense" as sending weapons and equipment.
The U.S. and U.K. and France only agreed to "assurances" of Ukrainian security........not "guarantees" as Ukraine requested.
That memorandum only says that the signers agree to:
"1. Respect the signatory's independence and sovereignty in the existing borders.[6]
2. Refrain from the threat or the use of force against the signatory.
3. Refrain from economic coercion designed to subordinate to their own interest the exercise by the signatory of the rights inherent in its sovereignty and thus to secure advantages of any kind.
4. Seek immediate Security Council action to provide assistance to the signatory 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".
5. Refrain from the use of nuclear arms against the signatory.
6. Consult with one another if questions arise regarding those commitments.[7][8]"
And.......then there's this:
"Regardless, the United States publicly maintains that "the Memorandum is not legally binding", calling it a "political commitment".[21]
Russia has violated that memorandum since 2014 and our response has only been sanctions and sending military equipment but no troops........it seems our level of response has been demonstrated.
So.......it IS possible that if Russia attacked Ukraine with tactical nuclear weapons we could increase that assistance to providing the Ukrainians with tactical nuclear weapons.......but that would be a profoundly dangerous step.
Budapest Memorandum - Wikipedia