When a nuclear power is directly attacked, do they ALWAYS use full retaliation?

Hell no, unless they are crazies who would do that? No one ever has responded with nuclear force yet. Of course those in control of Iran are crazies, 12ers, and they want the end of the world through war so they are scary with nukes. Probably a hell of a lot worse than scary, incredibly dangerous like dropping a bottle of nitro on the cement floor you are standing on dangerous.