The Order of Rank is only possible with the emergence of the self-styled warrior-god. For it is he, as an exalted being who reduces the vanquished to slavery.
Prisoners of war are the first slaves.

From the Sword Strong derives the hierarchy;

Warrior king and the warrior caste,
Priests [who worship the gods made in the image of the warrior band],
Peasant workers [who are of the race], and
Slaves [aliens captured in war].