Budapest memorandum (Будапештский меморандум) is treaty between Russia, Ukraine, UK and USA about nuclear weapon. According to this memorandum, Ukraine becomes non–nuclear country, while Russia, USA and UK respect the integrity of Ukraine.
The Budapest memorandum had been signed in Budapest 1994.12.05 and broken by Russian troops 2014.02.20 in Ukraine with beginning of the Russian invasion into Ukraine.
The text of Budapest memorandum is copypasted below from Wikisource  in order to simplify the referencing.
Memorandum on Security Assurances in connection with Ukraine's accession to the Treaty on the Non-Proliferation of Nuclear Weapons
Budapest, 5 December 1994
The United States of America, the Russian Federation, and the United Kingdom of Great Britain and Northern Ireland,
Welcoming the accession of Ukraine to the Treaty on the Non-Proliferation of Nuclear Weapons as a non-nuclear-weapon State,
Taking into account the commitment of Ukraine to eliminate all nuclear weapons from its territory within a specified period of time,
Noting the changes in the world-wide security situation, including the end of the Cold War, which have brought about conditions for deep reductions in nuclear forces.
Confirm the following:
The United States of America, the Russian Federation, and the United Kingdom of Great Britain and Northern Ireland, reaffirm their commitment to Ukraine, in accordance with the principles of the CSCE Final Act, to respect the Independence and Sovereignty and the existing borders of Ukraine.
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.
The United States of America, the Russian Federation, and the United Kingdom of Great Britain and Northern Ireland, reaffirm their commitment to Ukraine, in accordance with the principles of the CSCE Final Act, to refrain from economic coercion designed to subordinate to their own interest the exercise by Ukraine of the rights inherent in its sovereignty and thus to secure advantages of any kind.
The United States of America, the Russian Federation, and the United Kingdom of Great Britain and Northern Ireland, reaffirm their commitment to seek immediate United Nations Security Council action to provide assistance to Ukraine, as a non-nuclear-weapon State Party to the Treaty on the Non-Proliferation of Nuclear Weapons, if Ukraine should become a victim of an act of aggression or an object of a threat of aggression in which nuclear weapons are used.
The United States of America, the Russian Federation, and the United Kingdom of Great Britain and Northern Ireland, reaffirm, in the case of the Ukraine, their commitment not to use nuclear weapons against any non-nuclear-weapon State Party to the Treaty on the Non-Proliferation of Nuclear Weapons, except in the case of an attack on themselves, their territories or dependent territories, their armed forces, or their allies, by such a state in association or alliance with a nuclear weapon state.
The United States of America, the Russian Federation, and the United Kingdom of Great Britain and Northern Ireland will consult in the event a situation arises which raises a question concerning these commitments.
This Memorandum will become applicable upon signature. Signed in four copies having equal validity in the English, Russian and Ukrainian languages.
- http://npubop.livejournal.com/2475593.html Если называть вещи своими именами, То сейчас идет война РФ против Украины. 2014-12-17 17:56:00.
- http://www.profi-forex.org/novosti-rossii/entry1008208512.html Эксперты назвали 12 фактов, как медаль "За возвращение Крыма" подставила РФ (7). 21 апреля 2014 г. 18:38.
- https://www.facebook.com/stopputinsaggression/photos/a.612763438803433.1073741828.612740355472408/700599566686486/?type=1&theater Stop Putin's Aggression. 2014.09.08.
- http://onpress.info/v-dnr-zayavili-chto-ukraina-vosstanovila-svoj-yadernyj-potentsial-foto-22342 В «ДНР» заявили, что Украина восстановила свой ядерный потенциал. 9.02.2015. Сторонники террористической организации «ДНР» и «ЛНР» утверждают, что Украина восстановила свой ядерный потенциал..
- http://gordonua.com/news/politics/yuriy-rybchinskiy-nas-obmanuli-znachit-my-imeem-pravo-vernut-yadernoe-oruzhie-187487.html Юрий Рыбчинский: Нас обманули, значит, мы имеем право вернуть ядерное оружие. 10 мая, 2017 17.01. Украина должна восстановить свой ядерный статус после того, как был нарушен Будапештский меморандум. Такое мнение высказал украинский поэт Юрий Рыбчинский.
- http://www.defenseone.com/threats/2017/05/russian-lawmaker-we-would-use-nukes-if-us-or-nato-enters-crimea/138230/ Patrick Tucker. The Western alliance’s troop buildup in Eastern Europe has Moscow spooked. May 28, 2017. The Western alliance’s troop buildup in Eastern Europe has Moscow spooked.// BRATISLAVA, Slovakia — Russia would be forced to use nuclear weapons in any conflict in which U.S. or NATO forces entered eastern Ukraine, a member of Russia’s parliament told an international gathering of government security officials on Sunday.// “On the issue of NATO expansion on our borders, at some point I heard from the Russian military — and I think they are right — If U.S. forces, NATO forces, are, were, in the Crimea, in eastern Ukraine, Russia is undefendable militarily in case of conflict without using nuclear weapons in the early stage of the conflict,” Russian parliamentarian Vyacheslav Alekseyevich Nikonov told attendees at the GLOBSEC 2017 forum in Bratislava, Slovakia.
2914.04.15. http://news.err.ee/v/politics/4133bae8-8afe-4d57-9a99-6f41c969911c 4/15/2014 2:00 PM. US Senator John McCain visited Tallinn as part of a tour of Baltic nations, Norway and Moldova, and was interviewed by ETV's "Välisilm" program on Monday. .. We should support Ukraine as Ukraine and the USA are related with the Budapest memorandum.."
2016.04.28, the project of special bill in support of Ukraine is suggested in the USA: https://democrats-foreignaffairs.house.gov/sites/democrats.foreignaffairs.house.gov/files/H.R.%205094.pdf https://www.govtrack.us/congress/bills/114/hr5094/text 114th CONGRESS// 2d Session// H. R. 5094// IN THE HOUSE OF REPRESENTATIVES// April 28, 2016 Mr. Engel (for himself, Mr. Kinzinger of Illinois, Mr. Levin, Mr. Fitzpatrick, Ms. Kaptur, Mr. Abraham, Mr. Costa, Mr. Weber of Texas, Mr. Deutch, Mr. Pompeo, Mr. Cicilline, Mr. Shimkus, Mr. Keating, Mr. Bilirakis, Mr. Cohen, and Mr. Ribble) introduced the following bill; which was referred to the Committee on Foreign Affairs, and in addition to the Committees on Financial Services, the Judiciary, and Ways and Means, for a period to be subsequently determined by the Speaker, in each case for consideration of such provisions as fall within the jurisdiction of the committee concerned// A BILL // To contain, reverse, and deter Russian aggression in Ukraine, to assist Ukraine’s democratic transition, and for other purposes. .. (a)Short title // This Act may be cited as the Stability and Democracy for Ukraine Act or STAND for Ukraine Act.