Niven, Larry

A cross-over conclusion to the Ringworld and Fleet of Worlds series finds the Puppeteers of the Fleet of Worlds targeted by rival war fleets after the disappearance of the Ringworld, prompting exiled Puppeteer Hindmost to draw on his knowledge of Ringworld technology to reclaim his power.

Invaders from another star, Snouts, launch a war of indescribable violence and demand servile surrender or death for all humans.

The empire of man has quarantined the alien Moties, but now the Moties must breed or die.