You can draw a straight line from the USSR breaking up, to Ukraine giving away their nuclear weapons via the Budapest Memorandum, to the invasion of Ukraine by

You can draw a straight line from the USSR breaking up, to Ukraine giving away their nuclear weapons via the Budapest Memorandum, to the invasion of Ukraine by Russia (which was a signatory). States are going to be less likely to give up their nuclear weapons, even with treaties, because treaties can be easily broken.