We all know, through centuries, what the causes of war are...

We all know, through centuries, what the causes of war are: nationalism, economic separation, different kinds of societies, my country and your country, my prejudice against your prejudice, my love of my country and your love of country, my leader and your leader, and so on. We have known this for millennia, and we are still at it. So I say: what has happened to human beings? They know the danger of nationalism but they still wave the flag.