Good question. I don't know the answer but speculate that it has been overtaken by:
-- the 1998-1999 Kosovo war, in which the U.S. and NATO used force without authorization from the UN (and hence contrary to international law) to detach Kosovo from Serbia, setting a precedent for the Russian recovery of Crimea from Ukraine.
-- Ukraine's abandonment of neutrality and US-encouraged application for NATO membership in 2008.
-- the 2014 US-supported anti-Russian coup in Kyiv.
Some would no doubt argue that all these changes render the Budapest memorandum's guarantees moot under the doctrine of RES SIC STANTIBUS.
Someone else may have a better answer.
Chas
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