I do think it's human nature. In our animal nature, we have the fight or flight response. When it comes to global struggles, these seem to prevail. I think our gut instincts kick in -- fight or flight?! -- and then some general says that war is the answer. Or a political leader says we have to ignore the problem because we cannot afford to get involved. And one gut reaction gets reinforced and we're either off to war or ignoring atrocities. It happens all the time.
I agree with you that many individuals have evolved past this. I really think that if people start acting peacefully and proactively in their own lives and this becomes the norm, we will eventually have peace between nations.
It has to start at the individual level, because government always follows the people, not the other way around.
However, I do think a Dept of Peace could be a positive thing for an otherwise fairly militaristic, greedy and self-absorbed nation. It could plant a seed.