https://euromaidanpress.com/2017/04/13/ukrainian-schoolgirls...
After all, Putin can move with impunity, because if we do anything, he nukes everyone, and the anwser to the margin-nuke question „Should we really start a nuclear armageddon for X?“ is always no, even if it‘s progressing from „Ukraine“ to „Romania“, „Italy“, „remaining EU countries“, „NYC“, „Washington State“, „Southern half of US states“, etc.
From a game-theoretical POV, any nuclear power is allowed to do anything as long as the marginal anything they are currently doing is small enough.