The point is not so much that Ukraine should not have agency, but that actions have consequences. In this case: if Ukraine decides to flirt with joining NATO, it will anger Russia to the point of an invasion. That doesn't mean Ukraine can't make this decision, but Mearsheimer would consider it a wrong one given the damage a Russian invasion would do, and something leaders in Ukraine and NATO should have steered Ukraine away from through clear and strong messaging to the Ukrainian population.
It depends a bit on the timing of the predictions, but apart from the Ukraine one, these were pretty obvious, no? I also don't know what Mearsheimer's success percentage is.
Also, in the Ukraine case: the argument seems to be very focused on NATO, but as e.g.
@oneniltothearsenal argued in the last few pages, that might be a very limited view. Ukraine is the last former Warsaw Pact or USSR country in Europe that isn't in NATO, and thus Russia's last chance to have wider influence on its western side. (Well, there's Moldova and Transnistria, but they're tiny. And Georgia if you consider it European.)
Ukraine may well have been in Russia's crosshairs anyway if it didn't follow Russia's political and economical path - which is a very poor outlook for any country and perhaps well worth suffering this war for. (Without meaning to diminish the enormous death and hurt and damage it's causing!) Then there's the unknown of Putin's personal convictions: has he really become a crazy megalomaniac or is this still brutal calculation thickly coated in nationalistic messaging?
Both aspects in my understanding are beyond Mearsheimer's geopolitical projections - which is why e.g.
@Raoul keeps saying that realists consider a too limited set of factors in their geopolitical thinking. Ultimately, that would mean that Mearsheimer may have arrived at the right conclusion almost accidentially through incomplete arguments.
But that cuts both ways, as Mearsheimer employs a 'what if' as well when he says that (the threat of) NATO expansion caused the invasion of Ukraine - cause the implication is that there would have been no invasion without it. I don't think I agree with that.
The problem is that the concept of joining NATO to some extent becomes a proxy for 'turning west'. You're right that NATO continues to exist because of Russia; but if all those Warsaw Pact and USSR countries had not joined NATO but had still become a part of the EU, I think chances are that Russia under Putin would still have invaded Ukraine if it had started flirting with becoming an EU country as well.
Plus again, this works both ways. NATO continues to exist because of Russia, but that's also because Russia continues to position itself in a way that makes countries want to have a NATO and become its member. In that sense, you could consider this relationship a kind of vicious cycle - although I do think I would put most of the blame for that dynamic, at least since about 2000, with Russia. I mean, what would Russia have lost if they had just ignored NATO and had taken on a much less aggressive tone overall, thus breaking the cycle?
But I would also suggest that the US (and maybe other countries) is probably very happy with this dynamic, as NATO is an important way to maintain its sphere of influence. So if Russia would have employed a very different international rhetoric, the US might have looked for ways to demonize it anyway - ultimately leaving Russia no choice but to take up the other position again and becoming part of that NATO-reinforcing dynamic again.