The political legitimacy of egyptian pharaohs, mesopotamian kings, and shang kings was closely tied to their perceived ability to use religious ritual and engineering to control one vital resource:

The political legitimacy of egyptian pharaohs, mesopotamian kings, and shang kings was closely tied to their perceived ability to use religious ritual and engineering to control one vital resource: water.

All the cities and civilizations were (and are) built near rivers or others water bodies. These people believed that the Gods controlled the seasons and other natural phenomena. They practised a lot of rituals to keep the Gods please so the conditions for living were optimal. The rulers led these rituals, as they ruled because of the grace of the Gods.

in 476 c.e. romulus, the last of the roman emperors in the west, was overthrown by the germanic leader odoacer, who became the first barbarian to rule in rome. the order that the roman empire had brought to western europe for 1000 years was no more.