Also, since everything is based on the bell curve, out of 7 billion people, there will be people who are born with the perfect storm of personality traits to start a war, and with the right (or wrong, more accurately) life experiences will start that war. Hitler was a perfect example. Statistically it has to happen.
And we are only aggressive because, socially, we are barely out of the trees. We have the ability to blow the world up many times over, yet are consumed by petty intolerance. Our social intelligence lags our technical intelligence considerably. This is the most dangerous time in any civilisations time.