The Budapest Memorandum on Security Assurances, signed in 1994, provided security assurances to Ukraine in exchange for its relinquishment of nuclear weapons, with commitments from Russia, the USA, the UK, and France to respect Ukraine's sovereignty and borders.
https://en.m.wikipedia.org/wiki/Budapest_Memorandum
https://en.m.wikipedia.org/wiki/Budapest_Memorandum
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The current regime's actions are yet another violation.