What things began to change once Kush separated from Egypt?

The three kingdoms were the Old, Middle, and New Kingdoms. The Ancient Egyptian civilization begins. The first Pharaoh of Egypt, Menes, united the Upper and Lower parts of Egypt into a single civilization.

While experts believe the low water in the Nile was the primary influence for the fall of ancient Egypt, during the later period of the empire's existence, a two - to three-decade period was marked by erratic flooding of the Nile, destroying crops and starving the people dependent upon them.