https://en.wikipedia.org/wiki/Nagorno-Karabakh_conflict is as good an explanation as any
The area has two interwoven groups of different ancestry and religions and both sides seem unable to live with the others - its the Balkans again.
The problems have gone on for years and will no doubt continue long after Mr Putin has departed the scene.
It doesn't help when either side turns to a bigger neighbour (Russia, Iran, Turkey etc) who decide to meddle for their own ends