Yeah. A lot of those groups were frequently warring. But that didn't stop with the development of city states, as Mongolian, Roman, Persian, England and the Ottoman marched on with war to build an empire, while those such as the Greek city states as well warred against eachother. The Romans called it civilization, Tacitus called it their slavery.
But today it's been nearly a century sense Europe has had any war amongst each other. That's very significant and strong evidence to the claim the world is getting less violent. Decentralized power had more to do with this than the development of permanent settlements.