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The undertaking referenced is the Budapest Memorandum, in which Ukraine agreed to give up nuclear weapons in its possession, in exchange for promises force would never be used against its territory, with Russia, the United States, and England signing as guarantors.

Not even Russia claims the agreement has been abrogated, but merely that the departure of Crimea from Ukraine resulted from internal political processes, not any use of force by Russia or anyone else.

The idea that a revolution in a country ends treaty obligations has no foundation whatever in international law; a revolutionary government may repudiate a previous regime's signature, but even that may not be recognized as relieving it of treaty obligations by other signatories. Similarly, a government may declare it is no longer bound by assurances given a previous regime, and this also may not be recognized as relieving it of obligations by anyone else.