I read Mearsheimer's "Tragedy of Great Power Politics", and I think it's one of the most important books ever written. It completely changed the way I think about what's going on in the world. That being said, I think on the topic of Russia and Ukraine, Mearsheimer is not consistent with his own theory (defensive realism). I completely understand why Putin is doing what he's doing, his actions are very well explained…
My take on it is: if you willingly push a bear in a tight corner, you have it coming when he starts biting and fighting.
Now, this is no valid excuse for me when it comes to nation states and their actions, but nations are just run by humans, aren't they?