Ukraine explicitly gave up the nukes in their possession in return for recognition as a sovereign nation. Neither Russia NOR the United States were willing to let Ukraine retain those weapons. If you're referring to the Budapest Memorandum as a "guarantee", it was anything but; extant records from that time explicitly show that the United States were not putting a guarantee of military assistance on the table. (This is also notwithstanding that the Memorandum in question also said there would be no attempts at violating Ukraine's political and economic integrity, and the West's hands certainly aren't clean in that respect.)
At any rate, the Memorandum's applicability is somewhat dubious after Ukraine's government was overthrown during Euromaidan, and the terms of the Memorandum itself would have been superseded by actual treaties like Minsk I and Minsk II.
Fast forward a few years and Russia takes Crimea.
Two decades is "a few years"?
Now, if the west does not help Ukraine,
If the West had not "helped" Ukraine, Russia and Ukraine would have agreed to a ceasefire within months of Russia's initial invasion. (Thank Boris Johnson for convincing Zelensky to not negotiate.)
And they will look at the history of Ukraine and think "western promises are worthless".
I think we already proved that with the case of Libya and Muammar Gaddafi. After the Iraq War, he decommissioned his own weapons programs and desired greater economic/military cooperation with the West, eventually getting to the point that Libya was no longer a state sponsor of terrorism. Yet, in 2011, NATO air strikes and a NATO-enforced no-fly zone signalled the death knell for Gaddafi following the Libyan Civil War. Years later, Libya is still a dumpster fire.
I think that incident, more than anything else, would have told other nations that getting rid of their own weapons programs to appease the West would not guarantee their help (as not even a decade had elapsed between Gaddafi decommissioning those weapons and Gaddafi getting ousted).
Memorandum on security assurances in connection with Ukraine’s accession to the Treaty on the Non-Proliferation of Nuclear Weapons. Budapest, 5 December 1994.
Certificate of registration of the Memorandum on security assurances with the United Nations Secretariat, 2 October 2014. (by Ukraine)
The Budapest Memorandum on security assurances never provided a promise of military assistance. It is clear from the actual text and parties have acted accordingly. Absolutely nobody volunteered non-required boot on the ground military support. It was a Memorandum, not a Treaty, and could not create any form of binding obligation.
No. 52241
____Ukraine, Russian Federation, United Kingdom of Great Britain and Northern Ireland and United States of America
Memorandum on security assurances in connection with Ukraine’s accession to the Treaty on the Non-Proliferation of Nuclear Weapons. Budapest, 5 December 1994.
Entry into force: 5 December 1994 by signature
Authentic texts: English, Russian and Ukranian
Registration with the Secretariat of the United Nations: Ukraine, 2 October 2014
Pursuant to the Budapest Memorandum on security assurances, there was a commitment that any arising situation would be brought before the UN Security Council.
4. The Russian Federation, the United Kingdom of Great Britain and Northern Ireland, and the United States of America 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 NonProliferation 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.
Ukraine was assured that UN Security Council consideration would be sought. It was sought. As everyone knew, Russia held a veto on any action by the Security Council.
Volodymyr Vasylenko, Ukraine’s former representative at NATO, who took part in drawing up the conceptual principles and specific provisions of the Budapest memorandum:
“the form and content of the Memorandum ... show that, unfortunately, the Budapest talks on giving Ukraine security guarantees did not eventually result in a comprehensive international agreement that creates an adequate special international mechanism to protect our national security.”
According to V. Vasylenko, “Ukraine had to give up nuclear weapons for it to become sovereign state and its independent status to be recognized all over the world.”
Ukraine's forgotten security guarantee: The Budapest Memorandum
DW News [German]
Date 05.12.2014
[Excerpts]
Twenty years ago, the Budapest Memorandum marked the end of many years of negotiations between the successor states of the Soviet Union and leading Western nuclear powers. Ukraine had a special place in the talks.After the collapse of the USSR in 1991, the eastern European country inherited 176 strategic and more than 2,500 tactical nuclear missiles. Ukraine at that point had the third-largest arsenal of nuclear weapons in the world after the United States and Russia.
But Leonid Kravchuk, then the president of Ukraine, told DW that was only formally the case. De facto, Kyiv was powerless.
"All the control systems were in Russia. The so-called black suitcase with the start button, that was with Russian president Boris Yeltsin."
Western pressure
Ukraine could have kept the nuclear weapons, but the price would have been enormous, Kravchuk says. Though the carrier rockets were manufactured in the southern Ukrainian city of Dnipropetrovsk, the nuclear warheads were not. It would have been too expensive for Ukraine to manufacture and maintain them on its own.
"It would have cost us $65 billion (53 billion euros), and the state coffers were empty," Kravchuk said.
Additionally, the West threatened Ukraine with isolation since the missiles were supposedly aimed at the United States. Therefore, "the only possible decision" was to give up the weapons, according to Kravchuk.
[...]
"Nowhere does it say that if a country violates this memorandum, that the others will attack militarily," said Gerhard Simon, Eastern Europe expert at the University of Cologne.
German journalist and Ukraine expert Winfried Schneider-Deters agrees, telling DW, "The agreement is not worth the paper on which it was written."
Cyber-Security: The Threats from Russia and the Middle East, Ferry de Kerckove, CGAI Fellow, Canadian Global Affairs Institute, (2019), at 2-3: (footnotes omitted)
On the latter point, the 1994 Budapest Memorandum on Security Assurances (not “guarantees”), although considered an important landmark, had a single purpose: to convince Ukraine to abandon its nuclear weapons in exchange for a commitment by the signatories to provide it with support: “1. 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 [Commission on Security and Cooperation in Europe] Final Act, to respect the Independence and Sovereignty and the existing borders of Ukraine.” The memorandum, although formally signed, is not a treaty. Indeed, “Although signatories ‘reaffirm their commitment’ to Ukraine in many passages, the memorandum requires them to do almost nothing concrete, in the event that Ukraine’s sovereignty – territorial or political – is violated. There aren’t any hard enforcement mechanisms.” Ukraine is the subject of the memorandum, rather than a full participant. Furthermore, according to Volodymyr Vasylenko, Ukraine’s former representative at NATO, who took part in drawing up the conceptual principles and specific provisions of the Budapest memorandum, “the form and content of the Memorandum ... show that, unfortunately, the Budapest talks on giving Ukraine security guarantees did not eventually result in a comprehensive international agreement that creates an adequate special international mechanism to protect our national security.”