Isaiah's intention was perhaps not so much to point to one particular time in history, but rather to show how a nation is affected when it is judged by God. He uses the first person to emphasize that it is the Lord speaking. The result of the judgment would be civil war—Egypt turning on itself. That being said, historical sources do indicate times when Egypt was divided into many smaller kingdoms.
2 J'armerai l'Egyptien contre l'Egyptien, Et l'on se battra frère contre frère, ami contre ami, Ville contre ville, royaume contre royaume.