I remember something a friend that got a masters in history said. Peoples often define themselves relative historical empires and their own contemporary enemies

I remember something a friend that got a masters in history said. Peoples often define themselves relative historical empires and their own contemporary enemies. As a result they have an incentive to play up both. I've wondered about the Spartans, they conquered and enslaved some of their neighbors. And kept them that was for 200 years or something. Probably meant they weren't able to manage anything more than that.