In 751 B.C., Kush invaded Egypt and, shortly thereafter, conquered it.

This occupation of Egypt lasted for over a hundred years, until both the Kushites and the Egyptians were defeated by an invading army from Assyria in 666 B.C.At that point, the Kushites returned to the safety of their homeland.
The Kushites and the Egyptians had been defeated by a superior technology.

While they were fighting with weapons made of copper and bronze, the Assyrians fought with iron.

Methods of smelting and working iron had been developed centuries before by the Hittites who lived in Asia Minor.

The use of iron spread across the Near East, becoming the basis for the Assyrian power.