My problem with Mearsheimer and the other realists is they have a very poor explanation why the status quo changes. Yes great power competition explains why tensions are high but does little explain why and when they spill over to war. Notably absent from this article is any discussion of "why now?". Commenters in the thread are saying that his 2015 article on the subject could be written last week. That's precisely…
Formal joining was on hold, but everything else was accelerating since 2014. Most notably arms shipments, military funding, and NATO/US troops in Ukraine. At some point, there are simply too many NATO arms in Ukraine to invade. Russian power is waning, and at the same time Ukraine-NATO integration was approaching the point of irreversibility.
In 2016, Ukraine was granted a NATO Comprehensive Assistance Package (CAP), comprising the advisory mission at the NATO Representation to Ukraine as well as 16 capacity-building programmes and Trust Funds.
In 2018, Ukraine was officially given an aspiring member status.
In 2021 NATO reaffirmed that “Ukraine will become a member of the Alliance with the Membership Action Plan (MAP)", which is the most traction on the topic since 2008.
From the Russian perspective the window to stop Ukraine was closing. Look at how much trouble they are having now. It would only have been worse with another year of arms shipments and training. Perhaps Russia miscalculated an they already waited too long.