It’s kind of interesting that Russia feels so "threatened" by NATO enlargement, but the experience over the past 30 years is that as NATO has gone through successive waves of enlargement, its conventional military capability has reduced and eroded.
In expanding, the experience has been that NATO members appear to feel "safer in numbers" and have used this perception of greater safety to actually spend the peace dividend, or refocus defense spending away from conventional forces aimed to counter an attack from Russia, to other defense spending aimed at countering other threats from global terrorism, or cyber-attacks.
This has resulted in the UK cutting the size of its army to 78,000 as per the latest defense review - the lowest since WW2. Germany has cut the number of main battle tanks from 2000 in 1991 to sub-200 now, and the U.S. only a few years ago withdrew its last MBTs from Europe. Set against this, Russia has 20,000 MBTs, 30,000+ APCs, several thousand planes, and thousands of artillery pieces.
Net - now Russia has overwhelming conventional military superiority in Europe at this time. And Russia knows this - it knows that NATO has little offensive conventional military capability in Europe and indeed would have to quickly resort to a nuclear defense if faced by an assault from the East.
So the idea that NATO enlargement is some kind of threat to Moscow is a joke - actually Russia should be promoting it, as the experience has been more members, less defense spending, less conventional military capability, and a smaller military threat to Russia. And the reality is that even if Ukraine joined NATO, it then likely would feel less inclined to spend more on defense. Instead, feeling un-nerved by the lack of NATO membership or support, President Zelensky this week announced investment in another 100,000 troops, to take the standing army to 350,000. It would not need to do this if it benefited from the NATO security guarantee - likely it would be cutting defense spending.
NATO tanks are not likely to roll on to Moscow, and even less likely if Ukraine joins NATO.
Well, one is left with the realization here that NATO is not really a threat to Russia, and NATO enlargement likewise, is not a threat in the conventional sense.
So why is Putin doing all this? Why is he escalating?
Two thoughts/conclusions here, if it is not all about NATO:
First, it's about Ukraine - remember here Putin's essay written over the summer. Putin ultimately wants Ukraine, as he sees it as core to Russia's own identity and great power status. He thinks that there is this historical Slavic brotherhood between Ukrainians and Russians, and since the collapse of the USSR, then with the Orange Revolution and EuroMaidan, Ukraine has now been set on a course West, away from Russia and over time risking to break this link with Russia. He feels he has to act now to stop this migration.
Second, it’s about ideas, not arms or weapons. Putin fears the underlying driver or concept of the EuroMaidan, which was a desire by Ukrainians for European and Western values - democracy, human rights, rule of law - which is a diametrically opposed offering to Putin's power vertical - the sovereign democratic/kleptocratic model.
Putin just hates colored revolutions as they provide the biggest threat to his own rule in Russia. And in this respect Ukraine is unfinished business - he lost the Orange and EuroMaidan revolutions.
And he wants to get revenge on those, prove they were mistaken, and that they don't deliver better governance and improved living standards for the populations of those countries. He believes his model is better, and he is determined to make EuroMaidan Ukraine fail, ultimately.
Hence, he cannot help himself from constantly intervening to undermine Ukraine's development.