What exactly is the "Budapest Memorandum"?
The "Budapest Memorandum on Security Assurances" is a diplomatic memorandum that was signed in December 1994 by Ukraine, Russia, the United States, and the United Kingdom.
It is not a formal treaty, but rather, a diplomatic document under which signatories made promises to each other as part of the denuclearization of former Soviet republics after the dissolution of the Soviet Union.
Under the memorandum, Ukraine promised to remove all Soviet-era nuclear weapons from its territory, send them to disarmament facilities in Russia, and sign the Nuclear Nonproliferation Treaty. Ukraine kept these promises.
In return, Russia and the Western signatory countries essentially consecrated the sovereignty and territorial integrity of Ukraine as an independent state. They did so by applying the principles of territorial integrity and nonintervention in 1975 Helsinki Final Act -- a Cold War-era treaty signed by 35 states including the Soviet Union -- to an independent post-Soviet Ukraine.