I'm in partial agreement with you here.
1. We did make "security assurances" to Ukraine in that 1994 Budapest Memorandum to get them to give up their nukes. (Which was probably stupid in hindsight.)
The help we've given them up to this point I think fulfills our obligation - the U.S. gave our word, and we came through in helping Ukraine through a couple months of invasion.
2. Putin has outright said - as recently as the night before this ridiculous war - that he wants to re-establish the USSR. It is in American citizens' interests to make sure that doesn't happen.
But that second point goes for Europe 10X more that it goes for the USA. Those laggards in Europe need to wake up and start fulfilling their basic security obligations to their citizens, rather than making ours responsible.
“We” didn’t, Bill Clinton did. That’s why it’s a memo and not a treaty.
But as you said, we have fulfilled the obligation anyhow.
There’s what Putin wants, and what Putin can do. They are mostly exclusive.
Russia is weaker and NATO is strong. There won’t be a new empire.
Russia’s demographics are poor. 1.5 births, replacement is 2.1…
We are in agreement about Europe fulfilling their duties.
An 'assurance' isn't the same thing as an 'obligation', for what it's worth. The only thing the Memorandum (not even a treaty) indicates the parties would do in the event of aggression against Ukraine was to consult the UN Security Council, which has indeed been done.