For nearly a millennium, Egypt's early pharaohs presided over a prosperous and wealthy state that built countless temples and palaces, enormous public works, and the famous Giza pyramids. Much of that prosperity depended on the regular inundations of the Nile River in a country that otherwise would be only desert. Then, around 2200 B.C., ancient texts suggest that Egypt's so-called Old Kingdom gave way to a disastrous era of foreign invasions, pestilence, civil war, and famines severe enough to result in cannibalism. In the past decade, climate data revealed that a severe and long-term drought afflicted the region during this...