^Well, "constantly" is a tricky word when dealing with a whole civilization. In retrospect it may look like the warfare was constant, but realistically there would've been cycles of increase and decrease, periods of relative stability between bursts of chaos, empires or alliances that managed to maintain a relative peace for a century or two at a time before they were torn down, etc. (The building of an empire is quite a bloody process, but once it's established it can keep things peaceful and stable within its borders for centuries. A subject of the Roman or Mongol Empire could walk from one end of the empire to the other without having to worry about being attacked or robbed.) No entire species is going to maintain an exactly constant and unvarying level of warfare every single century for their entire recorded history. So I'd say it's plausible that there could have been periods when conditions were stable enough to allow that kind of progress.