Pandora's Legions — Christopher Anvil

IS IT TOO LATE TO NEGOTIATE A SURRENDER?

The Centran Empire was both benevolent and complacent. Benevolent, even as it overwhelmed every new inhabited planet it encountered with its military might -- but once the planet was pacified, its inhabitants were uplifted to Centran standards of living and the benefits of membership in the Empire. Complacent, because the Empire had long been expanding without encountering more than minor obstacles. And then they came across Pandora's Planet, that is, Earth ....

In spite of the Centran superiority in technology, the conquest of Earth took over three months, when the invaders were used to wrapping things up in less than two weeks. But "conquest" might be too optimistic a term to use, since the locals were still waging guerrilla warfare and sabotage, and the Centrans were hanging on by their equivalent of fingernails. And then the Centran scientists tested a group of humans and found that the humans were more intelligent than the Centrans.

One Centran leader had an idea that might save the Centrans from being ignominously kicked off the planet again. Since the humans were so good at war, why not put together teams of humans to go out to the fringes of Centran space and handle planets that were proving difficult? The teams performed magnificently -- but back in the heart of the Empire, humans and their weird new ideas were spreading like wildfire, and if something weren't done soon, the humans would end up running the former Centran Empire....