"on the tenth day after the surrender of the walled city of memphis, cambyses [a king of persia before darius] took psammenitus king of egypt...to insult him...he tried psammenitus' spirit, as i shall show. he dressed the daughter of the king as a slave and sent her out with a pitcher to fetch water, together with other girls from the families of the leading men, dressed like the daughter of the king. ...after the water-carriers had passed by, cambyses next made psammenitus' son go out before him with two thousand egyptians of the same age, all with ropes bound round their necks and bridle-bits in their mouths; they were led out to be punished for those mytileneans who had perished with their boat at memphis; for such was the judgment of the royal judges, that every man's death be paid for by the deaths of ten noble egyptians. when psammenitus saw them passing and perceived that his son was being led out to die, and all the egyptians who sat with him wept and showed their affliction, he did as he had done at the sight of his daughter." - herodotus, the histories
although grisly, this is human conquest after two revolutions. the first revolution was slavery. it's not obvious that people who want you dead can be rendered harmless without wholesale slaughter. this is why god in the bible says to the israelites: "kill them all; be on the safe side." i imagine it takes a fair amount of experimentation to figure out how to keep slaves from offensive weapons, keep them from organizing, and break their spirit but not so much they are useless to their masters.
the next revolution is happening in the persian empire. "we don't need to enslave everyone. as long as people give us earth and water (tokens of surrender), behave as new members of our empire, pay taxes, and maybe provide soldiers when needed, we can let them be." the kinder, gentler persian empire only makes examples of you (and your family) if you resist.
and you can bring that up to modern times. the empires are certainly not going to tolerate rebellion, but they've further realized you don't need to torture people and slaughter their loved ones (for the most past). enough to condemn terrorism, put surveillance on suspects, and apply prison as necessary. perhaps eventually this will give way to an even softer methods of maintaining control.
(and credit where due, these ideas are mostly based on dan carlin's podcasts which are mostly based on lots of history books i have not read. http://www.dancarlin.com/hardcore-history-56-kings-of-kings/ if you have, like, 8 hours free.)