See if this helps:
https://en.wikipedia.org/wiki/Nuclear_weapons_and_Ukraine (USe thius to access the other links.)
After the dissolution of the Soviet Union, Ukraine held about one third of the Soviet nuclear arsenal, the third largest in the world at the time, as well as significant means of its design and production.
Dahlburg, Decemb. "Ukraine Votes to Quit Soviet Union : Independence: More than 90% of Voters Approve Historic Break with Kremlin. The President-elect Calls for Collective Command of the Country's Nuclear Arsenal". LA Times. Retrieved April 15, 2014.
Formally, these weapons were controlled by the Commonwealth of Independent States.[4] In 1994 Ukraine agreed to destroy the weapons, and to join the Treaty on the Non-Proliferation of Nuclear Weapons (NPT).[5][6]
Hanley, Jeremy (June 22, 1993). "Nuclear Weapons". Hansard. UK Parliament. Column 154. Retrieved September 9, 2018. The Minister of State for the Armed Forces (Mr. Jeremy Hanley): ... Some weapons are also possessed by Ukraine, Kazakhstan and Belarus, but these are controlled by the Commonwealth of Independent States.
William C. Martel (1998). "Why Ukraine gave up nuclear weapons : nonproliferation incentives and disincentives". In Barry R. Schneider, William L. Dowdy (ed.). Pulling Back from the Nuclear Brink: Reducing and Countering Nuclear Threats. Psychology Press. pp. 88–104. ISBN 9780714648569. Retrieved August 6, 2014. There are some reports that Ukraine had established effective custody, but not operational control, of the cruise missiles and gravity bombs. ... By early 1994 the only barrier to Ukraine's ability to exercise full operational control over the nuclear weapons on missiles and bombers deployed on its soil was its inability to circumvent Russian permissive action links (PALs).
Alexander A. Pikayev (Spring–Summer 1994). "Post-Soviet Russia and Ukraine: Who can push the Button?" (PDF). The Nonproliferation Review. 1 (3): 31–46. doi:10.1080/10736709408436550. Archived from the original (PDF) on August 8, 2014.
In exchange for giving up its nuclear weapons, Ukraine received financial compensation, as well as the security assurances of the Budapest Memorandum.[9]
Budjeryn, Mariana (September 1, 2016). "Was Ukraine's Nuclear Disarmament a Blunder?". World Affairs. 179 (2): 9–20. doi:10.1177/0043820016673777. ISSN 0043-8200. S2CID 151341589.
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https://www.rferl.org/a/ukraine-explainer-budapest-memorandum/25280502.htmlWith tensions rising in Crimea and pro-Russian forces controlling the peninsula's main airports, 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?
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.
Which principles in the Helsinki Final Act, reiterated in the "Budapest Memorandum," are relevant to the current situation in the Crimea?
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."
Is there anything legally binding about the "Budapest Memorandum" regarding Russia's obligations to respect Ukraine's territorial integrity?
"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.