About the Native Americans... many tribes were very warlike. Typically territories were dominated by the aggressive use of force. The limitations imposed by a Stone Age technology and subsistence lifestyle made a singular empire improbable...but force still ruled the day.
Hm, note that I said "since BEFORE the Roman Empire". I was also thinking of the Persians, Babylonians and Assyrians, not to mention the Chinese Empire and so on.
As for humanity learning better, its a nice dream but unlikely.