Budapest Memorandum

The United States of America, the Russian Federation, and the United Kingdom of Great Britain and Northern Ireland, reaffirm their obligation to refrain from the threat or use of force against the territorial integrity or political independence of Ukraine, and that none of their weapons will ever be used against Ukraine except in self-defense or otherwise in accordance with the Charter of the United Nations.

-- Article 2 of the Budapest Memorandum on Security Assurances, a treaty signed on 5 December 1994