NATO is the violent village bully who's starting to get a little too old and frail to keep at it, and the rest of the villagers are starting to give each other knowing looks in the pub while the old bully starts into his usual heavy drinking.
A very gradual, symmetrical escalation is what a sane nuclear-armed country would do in this scenario. Ideally with diplomatic communication about "this is what we are doing and why we are doing it."