The Vikings conquered a lot of people, and stole a lot of stuff and slaves without writing either. If you want to see a traditional society where literacy isn't a big deal, you don't have to look any further than the Taliban. Only about half of the people in Afghanistan can read and write, and it's about to be less since the Taliban is denying school to women. With illiteracy, might makes right since there are no written laws or ways for a man to distinguish himself without being a badass.