If Putin's an imperialist, why does NATO matter?

In the debate over why Russia invaded Ukraine, I place myself in the camp who believe NATO expansion was a key factor. Call this the “Mearsheimer camp”, after its foremost champion, John Mearshsimer. I was therefore disappointed – while watching a recent debate on the subject – to hear Mearsheimer say the following:

The conventional wisdom, which I’m sure all of you have heard ad nauseam, is that Vladimir Putin is responsible for this war, Vladimir Putin is an imperialist, he’s either trying to create a greater Russia or he’s trying to recreate the Soviet Union. And what’s going on here is that Ukraine is a country that he wants to conquer and incorporate into Russia; he wants to absorb it. There is absolutely no evidence to support that argument.

Why disappointed? Because it’s disingenuous to claim there is “absolutely no evidence” that Putin wants to conquer Ukraine and incorporate it into Russia. There’s actually quite a bit of evidence. And as the foremost champion of the “Mearsheimer camp”, going around dismissing such evidence makes the rest of us look bad.