Ah, you have hit the nail on the head. Almost all of civilization was built through slavery, prior to the industrial period. Agriculture to feed the people of one's country required very extensive manpower requirements. To free others to create the infrastructure of nation's required that others were enslaved to provide the manpower necessary to feed nation's. For thousands of years, all of the great civilizations employed slavery. Even though it wasn't called slavery, in Europe during the years that Kings ruled by divine right, the serfs and peasants belonged to the nobility that owned the land and could not leave the land.