Circumstances didn't change though. Ukrainian borders were guaranteed by Russia and US (and some European countries too I think) unconditionally. Ukraine didn't attack Russia, quite the opposite. And as per my first post, in the first attack Russia didn't even pretend they had a cause - they pretended it wasn't them.
> treaties [...] are worth nothing without the will and power to enforce them
Well, precisely. Russia has demonstrated time and again that they don't honor agreements. Any agreement where you give Russia something today in exchange for something tomorrow just doesn't work.
Circumstances did not change? I am not too familiar with those treaties but I assume you are referring to the treaties in the context of Ukraine surrendering Soviet nuclear weapons at the end of the Cold War, correct? At that time the Soviets almost certainly did not consider the possibility of their neighbor becoming a NATO member and potentially ending up with US troops and weapons at their border. Circumstances most certainly changed since then.
If we're talking about the Budapest Memorandum, cirumstances possibly changing are actually part of the treaty. The memorandum conditions the use of force against Ukraine, and makes provisions for what would happen if a party to the treaty or one of its allies attacks another party to the treaty (second and fifth point, one is about conventional response, the other about nuclear response):
> [The parties shall] refrain from the threat or use of force against the territorial integrity or political independence of the signatories to the memorandum, and undertake that none of their weapons will ever be used against these countries, except in cases of self-defense or otherwise in accordance with the Charter of the United Nations.
If memory serves right, Ukraine didn't invade Russia, and Russia is not under a UN mandate.
That's the treaty. It was unconditional and unlimited in time, and Ukraine gave up something real for that security. Even if the rest of your argument holds, the lesson is there: don't give up things for promises from Russia. Today Ukraine would certainly rather have the nukes than a piece of paper.
As for the rest of it... As said, Ukraine wasn't really on track for NATO membership. But even if it was, this is all hyperbolic speech from Russia. For one, they themselves have no issue having lots of weaponry right on the border, in Kaliningrad and Belarus. For two, they made so much noise about Ukraine, well, basically not being ruled out of NATO, but when Finland, with a much longer border and more money actually joined NATO, they whimpered some half hearted threats and were done with it. Didn't seem to care much one way or the other.
> treaties [...] are worth nothing without the will and power to enforce them
Well, precisely. Russia has demonstrated time and again that they don't honor agreements. Any agreement where you give Russia something today in exchange for something tomorrow just doesn't work.