Anvil of Stars

Prolog

At the end of The Forge of God, the Earth is dead, murdered by self-replicating spacefaring machines. A few thousand humans have been saved by other robots, machines sent by the Benefactors to defend primitive worlds and civilizations from the depredations of planet-killing probes. The Benefactor machines have succeeded in wiping out these probes within the solar system, but not before Earth’s total destruction.

Kept aboard a huge Central Ark while Mars is made ready for their habitation, the humans are informed of the Law, a galactic code that governs the behavior of civilizations. The Law demands that civilizations which make self-replicating killer machines be punished—with extinction. Humans must carry out this punishment, with the help of the Benefactors. Younger occupants of the Central Ark volunteer, and their journey begins.

This is how the balance is kept.