It requires only equal or greater response if the country is attacked.
It's completely within the doctrine to respond to a single nuke detonated by Putin on Ukrainian soil targetting Nato military forces, with a single Nato nuke, also on Ukrainian soil targetting Russian military forces.
https://en.m.wikipedia.org/wiki/Mutual_assured_destruction
It requires only equal or greater response if the country is attacked.
It's completely within the doctrine to respond to a single nuke detonated by Putin on Ukrainian soil targetting Nato military forces, with a single Nato nuke, also on Ukrainian soil targetting Russian military forces.