Egypt is humanity’s longest contiguous civilization – lasting some three millennia – for good reason. The Sahara protects the Egyptians from most directions, while the Nile provides them with the magic mix of reliable water supplies and seasonal soil and fertilizer. After the Egyptians finished uniting the Lower Nile in the 31st century BC, they were the absolute masters of their world and could get down to the serious business of celebrating their culture. The pyramids of Giza (above) were one glorious result.
But a safe and secure Egypt also was a stagnant one. After unification, there were no stresses upon the Egyptian system that required much attention. Technological advancements in metallurgy, agriculture, and social structures simply stopped. The only industry to become more sophisticated was monument building, which vividly demonstrated how amazing your world could become when fueled by a bottomless supply of disposable labor.
One of many results was slavery. And while slaves might be ideal for growing wheat or stacking up giant blocks of rock, it is fairly difficult to motivate them to resist invaders who target the slaves’ taskmasters. Once the rest of the world developed snazzy new technologies that could bypass the desert – wooden sailing ships and domesticated camels being the top two – Egyptian authority over its homelands simply crumbled.
For more on how geography impacts the Egyptian condition, see Chapter 2 of The Accidental Superpower.