[On the Conquests of the Medio-Achaemenid Sovereigns of Persia]
A) Actually, the Achaemenids of Persia were not able to fully integrate and subsume Egypt into their greater empire. Although Egypt was occupied by the Persians through the expeditions of Cambyses II who was the immediate successor of Cyrus II 'the Great', Persian hegemony and sovereignty over Egypt was intermittent. During the last years of the reign of Darius I 'the Great' Egypt successfully repelled the Persian garrisons and installed a native Egyptian on the Pharaonic throne.
B) In addition, the Achaemenid despots and sovereigns were not able to exert any effective control over the Scythian tribes who incessantly mounted minor expeditions on the fringes of the Persian Empire. Even the great Achaemenid despot Darius I 'the Great' could not fully defeat or repel the Scythian tribes.
C) Finally, the eastern regions and satrapies of the Persians [Bactria, Ferghana, Arachosia, Gandhara, Tapuria, Traxiane, Hyrcania, Sogdiana] were maintained only with great difficulty under Darius I 'the Great'. With the successive line of weak, decrepit and imperious rulers as well as rebellious satraps and the rise of numerous usurpers and claimants to the Achaemenid throne, the eastern territories and satrapies began to disintegrate and fragment eventually breaking away from Persian hegemony.