Second strike

From Freepedia

In nuclear strategy, second strike capability is a country's assured ability to respond to a nuclear attack with powerful nuclear retalliation against the attacker. It counters a first strike nuclear threat and can support a no first use nuclear strategy.

Second strike could be part of a mutual assured destruction defence strategy, though it may be a lower level minimal deterrence response.

Submarine-launched ballistic missiles are the classic, but very expensive, method of providing a second strike capability, though it needs to be supported by a reliable method of identifying who the attacker is.