Let’s take a look at a few slides from this lecture that Mearsheimer gave back in 2015 to get a bit of background on the subject. Mearsheimer is certainly not pro Russian in any sense, and a proponent of US global hegemony. First, here’s the demographic breakdown of Ukraine:
As we can clearly see from the voting patterns in both elections, the country is divided exactly across the current line of conflict. Furthermore, a survey conducted in 2015 further shows that there is a sharp division between people of eastern and western Ukraine on which economic bloc they would rather belong to:
And we can see a few interesting facts about Crimea in a US government study. First thing to note is that it was never part of Ukraine proper. US government referred to it as the Autonomous Republic of Crimea. Second thing to note is that majority of the people in Crimea do not consider themselves Ukrainian, and the biggest demographic considers themselves Russian: