That was my question. It seems so counter-intuitive that I would like to see the evidence. Frankly, my own opinion is that killing technology just got better, and mankind enthusiatically adopted the new methods.
Who decided that everyone was so totally peaceful prior to 1500?
My recollection is that China had major conflicts around 2,000 BC. Also the Hititites or Assyrians were doing a lot of fighting around then. The Egyptians had their innings with the Nubians as well. Additional catastrophe info. With the draining of the Iraq marshes by Saddam Hussein, there has been discovered a meteor crater about 2 miles in diameter, with the age about 4,000 years old. It would have really messed up the neighborhood, and triggered all kinds of conflict.