That depends on which period you are asking about.. . The period usually referred to as the "Empire Period" in Ancient Egyptian history is the New Kingdom, especially the 18th and 19th Dynasties.. There is no single reason for the retraction of Egyptian control over territories in Nubia and southwestern Asia following the mid-20th Dynasty. It may partially have been due to a series of extremely long reigns by kings such as Ramses II of the 19th Dynasty which, while stable and fairly successful while they lasted, created something of a crisis when an entirely new person came to the throne - someone perhaps not entirely prepared to become king and who was himself quite old at the time, thus leading to a series of short-lived kings.. There was also a movement of people called the Sea Peoples throughout the broader Near East at the time which seemed to have contributed to the fall and/or attirition of several empires, including that of the Hittites as these fairly militaristic people invaded coastal areas, apparently looking for a new home en masse. . Egypt also had problems with raid by desert dwellers to the west and apparently there was an increasing problem in the rise of the power of the priesthood of Amun at Thebes at the expense of the power and prestige of the king, usually at his residence in the north at Memphis.