That’s breathtakingly untrue. I know it’s the sensational popular view of historical warfare but it’s just not true. Generally the worst thing that would happen is to be enslaved. But as time goes on and we develop different power structures after the Romans, decapitation of the government becomes far more preferred. So there’s a big battle, the loser leader gets killed, and the remaining nobles swear loyalty to the new leader. Trained people are simply too valuable to kill out of hand.

Of course we do have documented instances of genocide and mass destruction. Nobody is saying it didn’t happen. It just wasn’t the normal mode of operation.