In a nuclear war, if your country has already been destroyed, why would you retaliate?

Imagine you’re on a ballistic nuclear submarine, and your homeland has just been annihilated by a nuclear first strike. What reason would you have to launch a second strike? The purpose of nuclear weapons is deterrence, to prevent attacks in the first place. But when there’s nothing left to defend, what’s the point of retaliating?

War is often described as the continuation of politics by other means.

But if your nation no longer exists, what political goal remains?

There seems to be no strategic or moral gain in retaliating, only more destruction.