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Budapest memorandum......
You're misrepresenting it - it was explicitly not a mutual defence agreement. As for interference; fortunately, that has turned into a rather grey area; Zelenskyy has been using the competencies developed in his previous life rather successfully - unfortnately it's not at all clear whether that will be enough., and obviously cannot undo the catastrophe that has already occurred.
Budapest memorandum was signed by the UK and USA as well so in my opinion it's enough for them to interfere in this war.
And if they don't - they shouldn't be trusted as well.