Mack Reynolds 
The Rival Rigelians [EPUB ebook] 

The team from Earth had the task of raising backward planets to the home world’s high level. The situation on Rigel was this:

‘The most advanced culture on Rigel’s first planet is to be compared to the Italian cities during Europe’s feudalistic yeas. The most advanced of the second planet is comparable to the Aztecs at the time of the Spanish conquest.’

‘These planets are in your control to the extent that no small group has ever dominated millions before. No Caesar ever exerted the power that will be in your collective hands. For half a century, you will be as gods and goddesses!’

But the Rigelians were themselves descended from the lost colonists of old Earth and they could learn their lessons as fast as they could be taught. In fact, they could even teach their teachers a thing or two. And therein lay the peril the professors from space never dreamed of.