In the 1994 Budapest Memorandum, the United States, Russia, and Britain committed “to respect the independence and sovereignty and the existing borders of Ukraine” and “to refrain from the threat or use of force” against the country. Those assurances played a key role in persuading the Ukrainian government in Kyiv to give up what amounted to the world’s third largest nuclear arsenal, consisting of some 1,900 strategic nuclear warheads.