When I finally looked closely at a map, the nato membership being a red line made a great deal of sense.
eastern ukraine is deep into the former USSR territory. letting nato airpower projection and defenses be there would be intolerable. in terms of conventional (non-nuclear) conflict modeling, it would possibly not be something they could counter acceptably. therefore they will not allow it.
Rather like Israel does when the Palestinians and Hamas store them. Also he's securing the areas of conflict by ridding the areas of conflict, such as the republics, of these creating the conflicts. Additionally appears to have dropped some missiles on airports and protecting the Black Sea port areas.
I'm not getting that he's bombing cities rather going after installation areas.