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 Central 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!
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Used availability for Christopher Anvil's Pandora's Legions
January 2002 : USA Mass Market Paperback
February 2002 : USA, Canada, UK Kindle edition