Alexander the Great encountered a people that he described as something like “Without food or clothing, their hair is matted, and they eat only what the shore washes up along the beaches.”

Alexander described them as beyond pitiable creatures, who were wholly without culture, or sophisticated language, weapons, or even tools. He said he passed them by without concern that his forces would be danger.

Which book will detail about highly unevolved tribal peoples, who nevertheless somehow managed to still live? Or, ancient peoples whose culture reversed?

Are there any histories of an army general not bothering to conquer another people because they were so pathetic that waging war on them was considered without courage / ungodly / unmanly / unheroic?

I'd guess nonfiction, but I am open to a fictional narrative too, if it devotes a good chunk of pages to encountering backward peoples