[defrost]

The US Strategic Automated Command and Control System (DDSACCS), which sends emergency action messages to nuclear forces.

Not entirely software .. but an example of a system not used in house .. or at all ... yet.

Greatest? Hmm. Hard to say.

[muzani]

Nuclear deterrence means that a nation can counter while/after being nuked, and it has to demonstrate this capacity confidently enough to deter any country that would nuke them.

It would be an amazing system. Cut the head off and up to 20% of the world is dead or injured, followed by the socioeconomic collapse due to destruction at this scale.