>>>the only thing Putin has to fear from NATO ABM systems, and NATO membership for Ukraine in general, is an end to their ability to conquer Eastern Europe militarily.
Around 9/11, the Russians were focused on Chechen insurgents and terrorists, and our relations with them thawed significantly: https://carnegieendowment.org/2001/10/24/u.s.-russia-relatio...
Later in the 2000s, Russia discussed NATO and EU membership but had generally been rebuffed. Still, they tried to maintain dialogue with NATO, even after slapping down Georgia: https://euobserver.com/news/27890. I think they considered their point made about keeping NATO in check, but we didn't get the message.
For about 15 years, Putin ran Russia without even possessing the capability for a major nation-state offensive in Europe. That changed with the reformation of the 1st Guards Tank Army, stationed west of Moscow, in 2014. The question so few people are investigating is "What are the things WE ("the West", generally speaking) have done to trigger the changes in Russian foreign policy? This is the introspection that I find deeply lacking. Everyone just chalks it up to "clearly he's a megalomaniacal madman! Nothing else needs be said!"