With tensions rising in Crimea and pro-Russian forces, Ukrainian Prime Minister Arseniy Yatsenyuk has called on Russia to "not violate the Budapest Memorandum." So what is the "Budapest Memorandum" and what does it have to do with Crimea?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.In the "Budapest Memorandum," Russia, the United Kingdom, and the United States promised that none of them would ever threaten or use force against the territorial integrity or political independence of Ukraine. They also pledged that none of them would ever use economic coercion to subordinate Ukraine to their own interest.They specifically pledged they would refrain from making each other's territory the object of military occupation or engage in other uses of force in violation of international law.All sides agreed that no such occupation or acquisition will be recognized as legal and that the signatories would "consult in the event a situation arises which raises a question concerning these commitments.""That's actually a much more complex question than it may sound. It is binding in international law, but that doesn't mean it has any means of enforcement," says Barry Kellman is a professor of law and director of the International Weapons Control Center at DePaul University's College of Law."The 'Budapest Memorandum' follows the Helsinki Final Act and essentially reiterates its provisions. There are confidence building measures and then a host of other broader obligations – primarily negative obligations. Don't interfere."Kellman concludes that there are a host of other sources of international law that oblige Russia to respect Ukraine's territorial integrity -- including the provisions of the CSCE treaty and the UN Charter.