On February 15 Professor John J. Mearsheimer gave a talk (video) about the Ukraine crisis. He starts out (at 3min) by explaining who has caused it:

The United States, mainly, and its allies are responsible for this crisis.
I recommend to watch it in full.

Like me and many other analysts Mearsheimer did not expect that a Russian move into the Ukraine would happen. Why the Russian government finally decided to take that step is not clear to me. I believe that Zelensky's lose talk about acquiring nuclear weapons for the Ukraine was one of the decisive factors. Who told Zelensky to come up with that?