I realized as people lamented the cuts in NATO forces in Europe over the past twenty years that anyone arguing that Russia is motivated by the threat posed by NATO must not care about conventional military balances. After all, the balance in Europe after the Soviet Union fell apart was strongly in Europe's favor with all of that spiffy American hardware recently proven in Iraq (1991) and all of that other stuff that NATO countries had accumulated. Ever since then, the Europeans have fully taken advantage of the peace dividend to cut their forces. The Germans have cut back so far that few of their planes and helos can fly.
So, this is scary to Russia? Well, the irony here is that Mearsheimer spent much of the 1980s researching and writing about force to space ratios and other ways to measure the balance of forces in Europe. If Mearsheimer stuck to his original expertise, he might be arguing something different now--that Putin is not threatened by NATO enlargement but rather encouraged by NATO weakness.
The second irony is that Mearsheimer is giving a heap of value to NATO. This is a man who blasted international institutions as being irrelevant--or false promises.* If institutions are epiphemenonal and otherwise not so important, why should enlargement matter? It shouldn't. Not to Mearsheimer. Same goes for the European Union--why should Putin care about this toothless organization. I mean: ESDP? Really? Europe has made little progress on developing as a security and defense organization, so how can it threaten Russia?
The third irony is that if Mearsheimer was being true to himself as an offensive Realist, he would be arguing that countries seek power and that Putin is doing that when the opportunity presents itself. So, the blame should not be on NATO enlargement but a combination of NATO weakness and Putin's thirst for power.
But for some reason, he wants to blame the US and NATO. Again, if he were consistent, he would focus on their weakness--the defense budget cuts, the pivot--and not enlargement. There is plenty of blame to go around. Of course, if one blames the west for being weak, then perhaps one has to blame Putin for being aggressive. And that, for whatever reason, just does not fit into Mearsheimer's narrative.
The funny thing is that Mearsheimer and I see NATO enlargement to Ukraine similarly in one respect--that it is an incredible commitment that should not be made. But we diverge over where to point the finger for the current crisis. I point to the east and he points to the west.
* That piece demonstrates how good Mearsheimer is good at trolling: 2700+ cites for a piece that treats its opponents as the thinnest of strawmen.