In the past a country could simply conquer territory and annex it, but that is no longer considered acceptable behavior, hence the need to stage a referendum. International law in the 21st century requires the consent of the governed, even if that consent is obtained at gunpoint through a fraudulent election.
P.S. I wouldn’t put too much weight in reports about Russia hitting any specific target, as there is growing evidence suggesting the Russia military is incapable of aiming at a given target and then hitting the target.