I don't understand how the two are in conflict. Taleb asserts that the drop in violence is not somehow inherent to human nature, while the Pax Americana is a time of relative peace guaranteed by americas military advantage. When that advantage drops and other countries contest it, then violence will go up, just like the Roman Empires temporary advantage in military power produced a time of relative peace. For the two to be in conflict you'd have to assert that the Pax Americana was an inevitable and inherent development in human nature, which I'm not aware of anyone claiming.