Since 2014, Ukraine has tried three times to convene consultations with the guarantor states of the Budapest Memorandum. Three times without success. Today Ukraine will do it for the fourth time. I, as President, will do this for the first time. But both Ukraine and I are doing this for the last time. I am initiating consultations in the framework of the Budapest Memorandum. The Minister of Foreign Affairs was commissioned to convene them. If they do not happen again or their results do not guarantee security for our country, Ukraine will have every right to believe that the Budapest Memorandum is not working and all the package decisions of 1994 are in doubt.
Budapest Memo is what binded Ukraine to not pursuing Nuclear Weapons. There's another quote from a Ukr Def Minister or something around this time that is more direct, but this seems like what really caused the invasion once the border build up had been ongoing.
https://en.wikipedia.org/wiki/Budapest_Memorandum
According to the three memoranda, Russia, the US and the UK confirmed their recognition of Belarus, Kazakhstan and Ukraine becoming parties to the Treaty on the Non-Proliferation of Nuclear Weapons and effectively abandoning their nuclear arsenal to Russia, and that they agreed to the following:
Respect the signatory's independence and sovereignty in the existing borders.
Refrain from the threat or the use of force against the signatory.
Refrain from economic coercion designed to subordinate to their own interest the exercise by the signatory of the rights inherent in its sovereignty and thus to secure advantages of any kind.
Seek immediate Security Council action to provide assistance to the signatory if they "should become a victim of an act of aggression or an object of a threat of aggression in which nuclear weapons are used".
Refrain from the use of nuclear arms against the signatory.
Consult with one another if questions arise regarding those commitments.
Ukraine will consider arming itself with nuclear weapons if it does not become a member of the NATO military alliance, the Ukrainian ambassador to Germany said.
"Either we are part of an alliance like NATO and contribute in this way to making Europe stronger ... or we are left with the other option, which is to arm ourselves," Ambassador Andriy Melnyk told Deutschlandfunk radio Thursday.
Kyiv would then "perhaps also consider its nuclear status," he said. "How else can we guarantee our defense?"
https://www.president.gov.ua/en/news/vistup-prezidenta-ukrayini-na-58-j-myunhenskij-konferenciyi-72997
Budapest Memo is what binded Ukraine to not pursuing Nuclear Weapons. There's another quote from a Ukr Def Minister or something around this time that is more direct, but this seems like what really caused the invasion once the border build up had been ongoing.
https://en.wikipedia.org/wiki/Budapest_Memorandum
https://uawire.org/zelensky-ukraine-may-reconsider-its-nuclear-status
this is from April 2021:
https://www.dailysabah.com/world/europe/ukraine-mulls-nuclear-arms-if-nato-membership-not-impending-envoy
thanks