If Russia were to use just one tactical nuke on Ukraine, countless nations of the world would rush to get nuclear weapons. For example the Russians have on several occasions threatened to nuke Turkey, they threatened to nuke Azerbaijan recently, they have threatened to invade various other central Asian Turkic states. This nuclear talk so far is perceived as no more then a dog barking, however if they did use one in Ukraine. It then becomes imperative for a nation like Turkey to have her own nuclear arsenal. Irrespective of how western nations would threaten Turkey not to get them, the risk of Russia nuking Istanbul or Ankara means Turks would have no choice.
Just like invading Ukraine caused many nations to rush to join NATO and to rearm, one tactical nuke will cause many nations to go nuclear.
So their big strategic advantage, is the threat of using nukes. But the moment they use one, they will within a decade have lost that advantage against the nations she threatens. Because no one knowing there is a serious chance they could be nuked, will not look to level the playing field.
