As to what the west could actually have done, it's things like "not elect Trump" or "not do Brexit" or "not have growing right wing movement in Hungary and nearby countries". The general fragmentation and disunity in Western culture over the last couple of decades is what allowed Putin to think he could get away with this. The Nato stuff is mostly just another false flag/excuse, or, at best, it's just one tiny part of the cause.
What does that have to do with anything? If anything Trump wanted to stop Nord Stream 2 and wanted Germany to arm itself, and the Hungarians have a deep-seated grudge against Russian imperialism. But none of that mattered in the end because none of it changed the fundamental calculus of "Russia has nukes, and they care more about Ukraine than the West does." We should have been honest about that, and allowed the Ukrainians to seek an arrangement with the Russians as best they could, in a way that doesn't end with civilians getting shelled. Instead, we gave them false hope with our tough-sounding rhetoric, which made a deal politically impossible for Ukranian leaders—and now people are dying because of our lies.
This comparison isn't valid. In that case, according to Mearsheimer, expansion of NATO to Ukraine is perceived as an existential threat to Russia.
https://en.wikipedia.org/wiki/NATO_bombing_of_Yugoslavia
Nato is a serious threat to Russia -- a threat the US would never tolerate on its own borders, it goes without saying (Cuban Missile Crisis).
i don't suspect Mearsheimer means that Joe Biden personally held onto Putin's face and moved his jaw and lips while lip-syncing to give the order to invade -- i suspect he just means the obvious - the US pushed Nato membership on Ukraine, knowing it/we/US would win with either outcome -- as long as it didn't escalate to nuclear war -- which it still might.
the possible outcomes were: 1) ukrainian nato membership, or 2) russian invasion of ukraine with all the pluses that brought for the US
once you realize the US played a central role in creating the conditions to start this war, you realize that they/the US/we have a central role to play in stopping this war.
there is no US anti-war movement anymore -- so Ukraine is getting smashed.
maybe the rest of the world is next.
Scott Alexander has a response to this that puts it very succinctly. The expansion of NATO is responsible for Russian aggression in the same way that a woman wearing revealing clothes is responsible for sexual assault. There may be a causal link but the blame lies firmly with the aggressor in both cases.