The two things that caused this war are:
1) NATO the "defensive" alliance tearing libya to shreds like a rabid dog, basically for sport. This signaled the start of Putin's paranoia.
2) NATO putting Ukraine on the path to membership and refusing to back off.
Joining a gang is dangerous. Ukraine paid dearly for trying to join ours.
2) NATO never put Ukraine on the path to membership, although as a result of this invasion they're changing their mind.
2) Please, don't embarrass yourself. Ukraine was explicitly on the path to NATO membership (as of 2008), and the place was crawling with NATO trainers and advisors.
NATO probably won't exist in a decade. The US is not that committed to European security, and nobody else is worth a spit.
2) This is what NATO itself said at the 2021 summit. It was crystal clear about its intention to continue the admission process for Ukraine.
If Russia cannot handle its neighbours making decisions as a sovereign nation, then thats there problem.
It's possible Russia would have accepted their membership if NATO hadnt underscored its status as an offensive alliance with the destruction of Libya in 2011. Sadly this confirmed the reality to every Russian that NATO is an imperial threat.