Ukraine gave up its nuclear arsenal, the world's third largest, when it signed the Budapest Memorandum. In return it was not to be messed with.
As you may know, it has indeed been messed with, although the answer to the question "who," the US or the Russian Federation, appears to be a function of either the questioner's belief systems, or his sources. (If you think the media propaganda and filter bubble phenomena associated therewith sound interesting, you'll want to see my next post).
Given that the memorandum's promises have been broken, Ukraine is now considering the obvious, reviving its nuclear capabilities:
Ukraine May Have To Go Nuclear, Says Kiev Lawmaker
by Tyler Durden, Zerohedge, 03/11/2014 (emphasis his)
"In the future, no matter how the situation is resolved in Crimea, we need a much stronger Ukraine," warned Pavlo Rizanenko, a member of the Ukrainian parliament, adding that "If you have nuclear weapons people don't invade you." It would seem tough for the West (and their START Treaty) to get behind a nation that, as USA Today reports, believes it may have to arm itself with nuclear weapons to enforce a security pact to reverse the Moscow-based takeover of Crimea. "We gave up nuclear weapons," (inherited from the Soviet Union) because of the 1991 agreement that The United States, Great Britain and Russia would "assure Ukraine's territorial integrity" but Rizanenko told his government today, "now there's a strong sentiment in Ukraine that we made a big mistake."
Providing the occasion for an interesting google filter bubble. Here it is, wriggling, translated and ready to be gawked at. An opportunity for you to familiarize yourself with the overall flavor of the following propaganda. Funny how one event or situation can be seen three ways, by a fawning media. Funnier how the information retrieved by the same google search -- or the same article in Wikipedia -- can be so different, depending on the language. I didn't even bother to proxy for this one. Didn't even clear cookies or cache. I think its a hoot.
English in blue, Russian, red, and Ukrainian, yellow. Why not. Google translations all the way; links to PDFs of the original language searches in their respective Google top-level domains (.com, .ru, .ua) appended.
First, the search for "Budapest Memorandum:"
About 4,740 results
Results: Approximately 12,400
Approximate number of results: 3770
Now Wikipedia. Notice how sparse the Enlish one is.
Budapest Memorandum on Security Assurances
From Wikipedia, the free encyclopedia
The Budapest Memorandum on Security Assurances is a political agreement signed in Budapest, Hungary on 5 December 1994, providing security assurances by its signatories relating to Ukraine's accession to the Treaty on the Non-Proliferation of Nuclear Weapons. The Memorandum was originally signed by three nuclear powers, the Russian Federation, the United States of America, and the United Kingdom. China and France gave somewhat weaker individual assurances in separate documents.
The memorandum included security assurances against threats or use of force against the territorial integrity or political independence of Ukraine as well as those of Belarus and Kazakhstan. As a result Ukraine gave up the world's third largest nuclear weapons stockpile between 1994 and 1996.
Following the 2014 Crimean crisis, the U.S. and U.K. both separately stated that Russian involvement is in breach of its obligations to Ukraine under the Budapest Memorandum, and in clear violation of Ukrainian sovereignty and territorial integrity. 
According to the memorandum, Russia, the U.S., and the UK confirmed, in recognition of Ukraine becoming party to the Treaty on the Non-Proliferation of Nuclear Weapons and in effect abandoning its nuclear arsenal to Russia, that they would:
Under the political agreement, the signatories offered Ukraine "security assurances" in exchange for its adhesion to the Treaty on the Non-Proliferation of Nuclear Weapons. The memorandum bundled together a set of assurances that Ukraine already held from the Conference on Security and Co-operation in Europe (CSCE) Final Act, United Nations Charter and Non-Proliferation Treaty. The Ukrainian government nevertheless found it politically valuable to have these assurances in a Ukraine-specific document. 
The Budapest Memorandum was negotiated as a political agreement. It refers to assurances, not defined, but less than a military guarantee of intervention. According to Stephen MacFarlane, a professor of international relations "It gives signatories justification if they take action, but it does not force anyone to act in Ukraine." In the U.S. neither the George H. W. Bush administration nor the Clinton administration was prepared to give a military commitment to Ukraine, nor did they believe the U.S. Senate would ratify an international treaty, so the memorandum was agreed as a political agreement.
China and France gave Ukraine somewhat weaker security assurances in separate documents. China's governmental statement of 4 December 1994 did not call for mandatory consultations if questions arose, just calling for "fair consultations". France's declaration of 5 December 1994 did not mention consultations.
1994 Crimean crisis
In 1990, Meshkov was elected as a deputy to the Supreme Council of Crimea (the republic's parliament). There he became the co-founder of the RDK Party (Republican movement of Crimea). In 1994, he stood at the helm of the electoral bloc "Rossiya" for the republican presidential elections, where he easily defeated in the second round of elections Mykola Bahrov who ran as an independent. At that time, Mykola Bahrov was the head of the Supreme Council of Crimea. During the second round of the 1994 Crimean presidential elections, Yuriy Meshkov won with 72.9 percent of the vote, and was elected as the republic's only president.
His main political platform was to facilitate much closer relationships with the Russian Federation up to the possible annexation of Crimea by Russia. Meshkov tried to initiate a military-political union with Russia and completely disregarded opinions of the Ukrainian government. He also tried to force the circulation of the Russian currency,[clarification needed] issue foreign passports to the Ukrainian population, and even transfer Crimea to the same time zone as Moscow. Due to the unforeseen resistance from the local opposition, Meshkov only managed to put his autonomous republic into Moscow's time zone. He also appointed the Russian economist Yevgeny Saburov as vice prime-minister; Saburov virtually became the head of the government. Other government officials[who?] disputed the appointment, arguing that Saburov could not hold the position because he did not have a Ukrainian passport. Yevgeny Saburov was forced to resign. After that he managed to paralyze the work of the Supreme Council of Crimea.
In 1995, the Ukrainian parliament scrapped the Crimean Constitution and abolished the post of president on March 17, 1995. After couple of preceding warnings in September and November 1994, on March 17, 1995 the President of Ukraine, Leonid Kuchma, signed the Law of Ukraine that scrapped the amended Crimean Constitution and some other Laws of AR Crimea, on the grounds that they contradicted the Constitution of Ukraine and endangered the sovereignty of Ukraine.
2003 Tuzla Island dispute
Main article: Tuzla Island
In 2003, Russian construction efforts were seen as an attempt to annex Tuzla Island off the Crimean coast of Ukraine.[dead link] The Russian threat to Tuzla led to the Ukrainian leadership appealing to NATO for consultations on security, as outlined in the 1997 NATO-Ukraine Charter, without result. The dispute led to negotiations over delimitation of the maritime borders. In a 2012 preliminary agreement, Ukraine and Russia agreed that Tuzla Island would be considered Ukraine's territory.[needs update]
2014 Crimean crisis
U.S. Secretary of State John Kerry speaks with British Foreign Secretary William Hague and Ukrainian Foreign Minister Andrii Deshchytsia after hosting the Budapest Memorandum Ministerial on the Ukraine crisis in Paris, France, on March 5, 2014.
Main article: 2014 Crimean crisis
In February 2014, unidentified troops seized or blockaded various airports, as well as other strategic sites throughout Crimea. Official Ukrainian sources have said that the troops are Russian, attached to the Russian Black Sea Fleet stationed in Crimea, likely placing Russia in violation of the Budapest Memorandum. The Russian Foreign Ministry has confirmed the movement of armoured units attached to the Black Sea Fleet in Crimea, but asserts that they are acting within the scope of the various agreements between the two countries. Other official Russian sources deny that the units in the area of Sevastopol International Airport, specifically, are attached to the Black Sea Fleet. Russia responded by supporting a referendum on whether the Crimea should join the Russian Federation. Russia announced the referendum was being conducted by 'local forces'. On March 16, a majority favoured Russian annexation of the Crimea. Ukraine vigorously protested the action as a violation of Article 1 of the Budapest Memorandum.
On 1 March 2014, the White House released a press release stating that Russia had breached its obligations to Ukraine under the Budapest Memorandum:
President Obama expressed his deep concern over Russia’s clear violation of Ukrainian sovereignty and territorial integrity, which is a breach of international law, including Russia’s obligations under the UN Charter, and of its 1997 military basing agreement with Ukraine, and which is inconsistent with the 1994 Budapest Memorandum and the Helsinki Final Act. The United States condemns Russia’s military intervention into Ukrainian territory.
—Office of the Press Secretary
In response to the crisis, the Ukrainian parliament has requested that the Memorandum's signatories reaffirm their commitment to the principles enshrined in the political agreement, and further asked that they hold consultations with Ukraine to ease tensions.
- Wikipedia, the free encyclopedia
Memorandum on Security Assurances in connection with the accession of Ukraine to the Treaty on the Non-Proliferation Treaty - an international treaty concluded on December 5 1994 between Ukraine, the U.S. , Russia and Britain on the non-nuclear status of Ukraine. The agreement contains provisions with Ukraine guarantees its sovereignty and security.
March 1, 2014, Russian President Vladimir Putin on the background of "Crimean crisis" in the Federation Council received permission to send troops to Ukraine, arguing it is "an extraordinary situation in Ukraine, threatened the lives of citizens of the Russian Federation, our compatriots, the personnel of the military contingent of the Russian Armed Forces Federation ". As of March 14 to send troops was not officially announced, but there is information about the numerous seizures of military facilities by armed men unmarked. The Ukrainian government believes that they are members of the Armed Forces  . March 4, 2014, Vladimir Putin said that he agreed with experts pointing out that, if we assume that there was a revolution in the Ukraine ( Evromaydan ), we should assume that in its territory, a new state. At the same time in relation to this (new) Russian State any binding documents did not sign  .
The legal status of
As of 2014, the contract is not ratified    . Ratification of the Budapest Memorandum (giving him the political and legal status) member countries support Volodymyr Lytvyn,  , for his revision performed Viktor Yushchenko .  According to Article 11 of the Vienna Convention on the Law of Treaties, "State's consent to be bound by a treaty may be expressed by signing the contract, exchange of instruments constituting a treaty, ratification of the treaty, acceptance, approval or accession, or by any other means if so agreed "  . The text of the memorandum stated, "This Memorandum will be applicable from the date of signing". 
In September 2009, former Secretary of the National Security and Defense of Ukraine Volodymyr Horbulin and Doctor of Political Sciences Alexander Litvinenko said that Ukraine should convene an international conference to prepare the agreement on security guarantees and replacement of the Budapest Memorandum, bring it to the state to guarantee security in Ukraine 1994, and other key geopolitical players. 
In Wikisource has texts on the subject
Budapest Memorandum [ ред.
- Wikipedia, the free encyclopedia.
Memorandum on Security Assurances in connection with Ukraine's accession to the Treaty on the Non-Proliferation of Nuclear Weapons - international agreement concluded December 5th 1994 , between Ukraine, USA , Russia and the UK on a non-status Ukraine . The agreement contains items that provide guarantees of Ukraine 's sovereignty and security.
Welcoming ... Ukraine's accession to the Treaty on the Non-Proliferation of Nuclear Weapons as a non-nuclear-weapon States, including Ukraine's commitments on removing all nuclear weapons from its territory in a timely manner, noting the changes in the world of security, including the end of the "Cold war "that created the conditions for deep reductions in nuclear forces, confirm the following:
1. The Russian Federation, United Kingdom of Great Britain and Northern Ireland and the United States reaffirm their commitment to Ukraine in accordance with the principles of the Final Act of the CSCE to respect the independence, sovereignty and the existing borders of Ukraine.
2 .... confirm their commitment to refrain from the threat or use of force against the territorial integrity or political independence of Ukraine, and that neither their weapons will never be used against Ukraine, except for purposes of self-defense or otherwise in accordance with the Charter of the United Nations .
3 .... reaffirm their commitment to Ukraine in accordance with the principles of the Final Act of the CSCE to refrain from economic coercion that seeks 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.
4 .... reaffirm their commitment to seek immediate action of the Security Council to assist Ukraine as a State Party to the Treaty on the Non-nuclear-weapon States in the event that Ukraine will become a victim of an act of aggression or an object of a threat of aggression using nuclear weapons.
5 .... on Ukraine confirmed their commitment not to use nuclear weapons against any state party to the Treaty on the Non-Proliferation of Nuclear Weapons that do not possess nuclear weapons, except an attack on their territory or dependent territories, their armed forces or their allies, by such a state with the state, which has nuclear weapons or related union agreement.
6. Russian Federation, United Kingdom of Great Britain and Northern Ireland , United States and Ukraine will be consulted in case a situation arises where there is a question regarding these obligations.
This memorandum will be applicable from the date of signing. Signed in four copies, which are equally authentic in English, Russian and Ukrainian languages.
In September 2009, former Secretary of the National Security and Defense Council of Ukraine Volodymyr Gorbulin and Doctor of Political Sciences Alexander Litvinenko stressed that Ukraine should convene an international conference to prepare the agreement on security guarantees and replacement of the Budapest Memorandum, bring to a conference of States to guarantee the safety of Ukraine in 1994, and other key geopolitical players  .
March 1, 2014, Russian President Vladimir Putin on the background of "Crimean crisis" was in the Federation Council (upper house) permission to invade Ukraine, arguing it "extraordinary situation in Ukraine, threatened the life of Russian citizens, our fellow citizens, personnel of the military contingent of the Armed Forces of the Russian Federation "  . As of March 2, the input of troops has not been officially announced, but there is information about the capture of numerous military installations armed men unmarked; Ukrainian authorities believe that they are the soldiers of the Armed Forces  .
Be seeing you.