The new states had to interact with their neighbors in new ways, which created even more disruptive paradoxes of development along their frontiers. They had to learn to manage these; when they got things wrong—as perhaps happened at Uruk in Mesopotamia around 3100 BCE and Taosi in China around 2300, and definitely happened in the West after 2200 and 1750 BCE—they collapsed in chaos. Each collapse coincided with a period of climate change, which, I suggested, added a fifth horseman of the apocalypse to the four man-made ones.
