Ukraine had committed to the Nuclear Non-Proliferation Treaty in 1991 when it signed the Lisbon Protocol. Ukraine agreed to accede as a non-nuclear nation.
Ukraine subsequently tried to add conditions to its ratification, which terms were deemed unacceptable by the United States and Russia. There was to be no sovereign state recognition for Byelarus, Kazakhstan, or Ukraine without their having acceded to the Nuclear Non-Proliferation Treaty (NPT).
Ukraine then rescinded its conditions and acceded as a non-nuclear state. It's accession as a non-nuclear state was required by Article 5 of the Lisbon Protocol.
All three former members of the USSR acceded to the NPT for the same reason — it was the cost of their gaining recognition as a sovereign state.
https://2009-2017.state.gov/documents/organization/27389.pdf
The Lisbon Protocol
ARTICLE IThe Republic of Byelarus, the Republic of Kazakhstan, the Russian Federation, and Ukraine, as successor states of the former Union of Soviet Socialist Republics in connection with the Treaty, shall assume the obligations of the former Union of Soviet Socialist Republics under the Treaty.
ARTICLE II
The Republic of Byelarus, the Republic of Kazakhstan, the Russian Federation, and Ukraine shall make such arrangements among themselves as are required to implement the Treaty's limits and restrictions; to allow functioning of the verification provisions of the Treaty equally and consistently throughout the territory of the Republic of Byelarus, the Republic of Kazakhstan, the Russian Federation, and Ukraine; and to allocate costs.
ARTICLE III
1. For purposes of Treaty implementation, the phrase, "Union of Soviet Socialist Republics" shall be interpreted to mean the Republic of Byelarus, the Republic of Kazakhstan, the Russian Federation, and Ukraine.
2. For purposes of Treaty implementation, the phrase, "national territory," when used in the Treaty to refer to the Union of Soviet Socialist Republics, shall be interpreted to mean the combined national territories of the Republic of Byelarus, the Republic of Kazakhstan, the Russian Federation, and Ukraine.
3. For inspections and continuous monitoring activities on the territory of the Republic of Byelarus, the Republic of Kazakhstan, the Russian Federation, or Ukraine, that state shall provide communications from the inspection site or continuous monitoring site to the Embassy of the United States in the respective capital.
4. For purposes of Treaty implementation, the embassy of the Inspecting Party referred to in Section XVI of the Protocol on Inspections and Continuous Monitoring Activities Relating to the Treaty between the United States of America and the Union of Soviet Socialist Republics on the Reduction and Limitation of Strategic Offensive Arms shall be construed to be the embassy of the respective state in Washington or the embassy of the United States of America in the respective capital.
5. The working languages for Treaty activities shall be English and Russian.
ARTICLE IV
Representatives of the Republic of Byelarus, the Republic of Kazakhstan, the Russian Federation, and Ukraine will participate in the Joint Compliance and Inspection Commission on a basis to be worked out consistent with Article I of this Protocol.
ARTICLE V
The Republic of Byelarus, the Republic of Kazakhstan, and Ukraine shall adhere to the Treaty on the Non-Proliferation of Nuclear Weapons of July 1, 1968 as non-nuclear weapon states Parties in the shortest possible time, and shall begin immediately to take all necessary action to this end in accordance with their constitutional practices.
ARTICLE VI
1. Each Party shall ratify the Treaty together with this Protocol in accordance with its own constitutional procedures. The Republic of Byelarus, the Republic of Kazakhstan, the Russian Federation, and Ukraine shall exchange instruments of ratification with the United States of America. The Treaty shall enter into force on the date of the final exchange of instruments of ratification.
2. This Protocol shall be an integral part of the Treaty and shall remain in force throughout the duration of the Treaty.
Done at Lisbon on May 23, 1992, in five copies, each in the Byelarusian, English, Kazakh, Russian, and Ukrainian languages, all texts being equally authentic.
FOR THE REPUBLIC OF BYELARUS:
P. KravchankaFOR THE REPUBLIC OF KAZAKHSTAN:
T. ZhukeyevFOR THE RUSSIAN FEDERATION:
A. KozyrevFOR UKRAINE:
A. ZlenkoFOR THE UNITED STATES OF AMERICA:
James A. Baker, III
The Breach: Ukraine’s Territorial Integrity and the Budapest Memorandum
In May 1992, the US, Russia, Ukraine, as well as Kazakhstan and Belarus, which also inherited Soviet nuclear weapons, signed a protocol making the latter three countries parties to START I. However, lest the accession to START I be interpreted as a commitment to reduce rather than eliminate nuclear arsenals, Article 5 of what became known as the Lisbon Protocol committed the non-Russian republics to accede to the NPT as nonnuclear weapons states (NNWS) “in the shortest possible time.”1010 Annex to the Strategic Arms Reduction Treaty of July 31, 1991, Signed by the USA, Russia, Ukraine, Belarus and Kazakhstan in Lisbon on May 24, 1992, 1992; reprinted in Victor Batiouk, Ukraine’s Non-Nuclear Option (New York: UN Institute for Disarmament Research, 1992)
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https://www.armscontrol.org/node/3289
Basic Timeline and Provisions:July 31, 1991: The United States and the Soviet Union sign START.
Dec. 31, 1991: The Soviet Union officially dissolves, delaying entry into force of START.
May 23, 1992: Russia, Belarus, Ukraine, Kazakhstan, and the United States sign the Lisbon Protocol.
• Under the protocol, all five states become parties to START.
• Belarus, Kazakhstan, and Ukraine promise to accede to the NPT as non-nuclear-weapon states “in the shortest possible time.”
July 2, 1992: Kazakhstan ratifies START.
Oct. 1, 1992: The U.S. Senate votes to give its advice and consent to ratification of START.
Nov. 4, 1992: The Russian State Duma refuses to exchange START instruments of ratification until Belarus, Ukraine, and Kazakhstan accede to the NPT.
Feb. 4, 1993: Belarus ratifies START.
July 22, 1993: Belarus submits its instrument of accession to the NPT as a non-nuclear-weapon state.
January 14, 1994: The Trilateral Statement is signed by U.S. President Bill Clinton, Russian President Boris Yeltsin, and Ukrainian President Leonid Kravchuk. It allows Ukraine to observe the transfer of weapons from its territory to Russia and the dismantlement of certain systems. It also commits Russia to send some of the uranium extracted from the returned warheads back to Ukraine for fuel.
Feb. 3, 1994: Ukraine ratifies START, rescinding conditions for ratification that it had issued in November 1993.
Feb. 14, 1994: Kazakhstan submits its instrument of accession to the NPT as a non-nuclear-weapon state.
Dec. 5, 1994: Ukraine submits its instrument of accession to the NPT as a non-nuclear weapon state.
• The five START parties exchange instruments of ratification for START, which enters into force.
April 24, 1995: Kazakhstan transfers its last strategic weapon to Russia.
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Ratification and Implementation:Belarus:
When the Soviet Union dissolved, the newly-established Republic of Belarus found itself in possession of roughly 800 total nuclear weapons deployed within its borders. Although Russia retained the warhead arming and launch codes, many worried that Belarus might attempt to take control of the weapons. Moreover, Belarusian President Alexander Lukashenko twice threatened to retain some weapons if NATO deployed nuclear weapons of its own in Poland. However, when a constitutional crisis erupted in November 1996, Lukashenko was finally compelled to finalize the transfers.
Minsk signed the Lisbon Protocol on May 23, 1992, ratified it on Feb. 4, 1993, and deposited its instrument of accession to the NPT as a non-nuclear-weapon state on July 22, 1993. By November 1996, all nuclear warheads in Belarus had been transferred to Russia.
Kazakhstan:
After gaining independence, Kazakhstan, with extensive U.S. technical and financial assistance, disposed of the strategic nuclear weapons that it inherited from the Soviet Union. Kazakhstan’s 1,410 strategic warheads were deployed on several different systems, including SS-18 ICBMs and cruise missiles carried by Bear-H bombers.
Kazakhstan’s parliament ratified START on July 2, 1992. All tactical nuclear weapons had been withdrawn to Russia by January 1992. The parliament approved accession to the NPT on Dec. 13, 1993, and deposited the state’s NPT instrument of ratification on Feb. 14, 1994. The last of the Kazakh-based strategic nuclear weapons were transferred to Russia by April 24, 1995.
Ukraine:
When the Soviet Union dissolved, Ukraine became the third-largest nuclear weapons power in the world behind the United States and Russia. Ukraine’s 1,900 strategic warheads were distributed among ICBMs, strategic bombers, and air-launched cruise and air-to-surface missiles. Although Ukrainian President Leonid Kravchuk signed the Lisbon Protocol on May 23, 1992, Ukraine’s process of disarmament was filled with political obstacles. Many Ukrainian officials viewed Russia as a threat and argued that they should keep nuclear weapons in order to deter any possible encroachment from their eastern neighbor. Although the government never gained operational control over the weapons, it declared “administrative control” in June 1992, and claimed in 1993 ownership of the warheads, citing the potential of the plutonium and highly enriched uranium they contained for creating peaceful energy.
A resolution passed by the Rada, the Ukrainian parliament, on Nov. 18, 1993, attached conditions to its ratification of START that Russia and the United States deemed unacceptable. Those stated that Ukraine would only dismantle 36 percent of its delivery vehicles and 42 percent of its warheads; all others would remain under Ukrainian custody. Moreover, the resolution made those reductions contingent upon assurances from Russia and the United States to never use nuclear weapons against Ukraine (referred to as “security assurances”), along with foreign aid to pay for dismantlement.
In response, the Clinton and Yeltsin administrations intensified negotiations with Kyiv, eventually producing the Trilateral Statement, which was signed on Jan. 14, 1994. This agreement placated Ukrainian concerns by allowing Ukraine to cooperate in the transfer of the weapons to Russia, which would take place over a maximum period of seven years. The agreement further called for the transferred warheads to be dismantled and the highly enriched uranium they contained to be downblended into low-enriched uranium. Some of that material would then be transferred back to Ukraine for use as nuclear reactor fuel. Meanwhile, the United States would give Ukraine economic and technical aid to cover its dismantlement costs. Finally, the United States and Russia responded to Ukraine’s security concerns by agreeing to provide security assurances upon its NPT accession.
In turn, the Rada ratified START, implicitly endorsing the Trilateral Statement. However, it did not submit its instrument of accession to the NPT until Dec. 5, 1994, when Russia, the United Kingdom, France, and the United States provided security assurances to Ukraine. That decision by the Rada met the final condition for Russia’s ratification of START and therefore subsequently brought that treaty into force.