Ukraine used to be a puppet Russian state but the west managed to switch it to a puppet westerns state a while back. Ukraine has long had big time corruption issues and pretty soon, the USA had lots of grubby little fingers in that pie. It could just be a matter of the Ukraine have been the favorite money grubbing corruption darling. And indirectly I would guess putin did not like having the wester infidels on his border, plus the Donbas regions actually WANTED to be separate from Ukraine and aligned with Russia, they asked for it, so that could easily explain why Putin went in to that region , they asked him to help them. The remaining question is why did he continue past those regions? My guess was he thought it would be easier than it was, that Ukraine would put up less fight and maybe accept it more than they have. Also I've heard that many russians have long considered the ukraine to be a rightful part of russia.