"diplomatic solution" at the point the US was involved likely would have meant seceding land to Russia. The US doesn't really have leverage to negotiate with Russia any better than they can barter with China over Taiwan.
Not to mention; I don't think anyone wanted to deploy Patriots in Ukraine for the same reason Russia didn't want to put the S-400 on their front-lines. Deploying a state-of-the-art weapon allows your opponent to study it and potentially find out if it's flawed or truly fearsome. If it weren't for Germany and Romania I don't think Ukraine would have American air defense at all.
There’s a good chance the war wouldn’t have happened if Russia were given assured perpetual access to Sevastopol base.
The moment Ukraine returned their nukes, they gave up any leverage they could have against Russian aggression. Also, as soon as Ukraine started warming up to joining NATO, Putin would need to choose between not following on with the threat that no former Soviet republic would be welcomed into NATO, or go to war before that former republic effectively joined.
This war happened, like most wars do, when diplomacy failed.
The threat that no former Soviet republic would be welcomed into NATO,
That ship had long since sailed. Former Soviet republics Latvia, Lithuania, and Estonia have been NATO members since 2004. Ukraine's NATO membership application had been effectively the table since 2008 (precisely as a de-escalatory gesture in his favor) in any case.
Putin would need to ...
Putin didn't "need to" do anything, at least nothing physically aggressive against its neighbors. He knew very well that NATO posed no substantial threat to Russia. A symbolic nuisance, at most. But no actual, physical threat to Russia's security.
He invaded because he chose to. He gambled that it would be a cakewalk, and win him a huge boast in prestige at minimal cost.
This was all a gigantic delusion of course, and that's how we got here.
The criticism section seems to explain why the treaty was on shaky grounds.
There is no right or wrong when it comes to diplomacy or war - whoever wins is who was retroactively right. I’m very sorry Ukraine and Russia were both dragged into this war because of two irresponsible and corrupt political systems.
I invite you to consider the possibility that, although individually we are powerless -- collectively and in an emergent fashion, we very much do create the reality of the world we live in (as concerns human and social constructs at least).
So if enough people are resigned to the idea that fascism (or its brother in terms of international relations -- the idea that the world is intrinsically amoral, and no one is ever really held to account for anything) is "just the way the world works" -- sooner or later, that's the system they'll be living under.
And in fact, this is one of the core mechanisms by which such oppressive systems take hold and gains their legs.
In more succinct terms (to the point of sounding trite): Another world is possible.
