(CNN) -- Russia's military occupation of Ukrainian territory on the Crimean peninsula constitutes a blatant violation of the commitments that Moscow undertook in the 1994 Budapest Memorandum on Security Assurances for Ukraine. The United States and United Kingdom, the other two signatories, now have an obligation to support Ukraine and penalize Russia. When the Soviet Union collapsed in 1991, Ukraine found itself holding the world's third largest nuclear arsenal, including some 1,900 strategic nuclear warheads that had been designed to attack the United States. Working in a trilateral dialogue with Ukrainian and Russian negotiators, American diplomats helped to broker a deal —the January 1994 Trilateral Statement — under which Ukraine agreed to transfer all of the strategic nuclear warheads to Russia for elimination and to dismantle all of the strategic delivery systems on its territory.
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Steven Pifer: In Ukraine occupation, Russia reneging on 1994 deal to leave it alone
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Budapest Memorandum let Ukraine shed nukes in exchange for its sovereignty, he says
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U.S., Russia, UK, Ukraine signed. Russia now reneging. West must penalize it, he says
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Pifer: Failure to support Ukraine could delegitimize future nuke deals, like with Iran
@placeholder negotiators —myself among them — discussed this point in detail with Ukrainian counterparts so that there would be no misunderstanding.