Quoted from The History of the Ancient World: From the Earliest Accounts to the Fall of Rome -  (Susan Wise Bauer) 

"The wanderers of the ancient world were gaining the edge over the organized kingdoms: “The foreign lands burst forth and scattered in the strife,” Rameses III wrote on his temple wall, “no land could stand before them. They laid their hands on countries as far as the circuit of the whole earth.”4 Most of the “whole earth,” as it happened, had been suffering through a decade of on-again, off-again drought, the same famine-producing dryness that probably sent the Libyans into the Delta. To thirsty wanderers, Egypt, with its always-watered lands, began to look like the world’s prize jewel. [emphasis mine] Not long into Rameses III’s reign, an organized alliance of invaders headed his way."

Ramses III reigned from 1186 to 1155 BCE. The falls of many cultures result from over indulgence and lack of resources to feed the people. Reading of the Russian drought, their embargo of grain, diminishing pure water resources in Africa, high food prices in the revolting Arabic sovereigns reminds me basics don't change. People need food and power needs greed. When each need reaches the extreme, revolt happens. The Wisconsin revolt is a simple example.

Just musing. Shall we talk of global warming?