Speaking sociologically, the structure of Egyptian society from sometime before 3,400 B.C., to 525 B.C., passed through four distinct phases or stages. During the first phase, the Nile Valley, which had been separated by tribal and/or geographical boundaries into a large number of more or less independent units, was consolidated, integrated and organized into a single kingdom. This working, functioning area (the land of Egypt) could provide for most of its basic needs from within its own borders. In a sense it was a self-sufficient, workable, liveable area. Egypt was populous, rich, well organized, with a surplus of wealth, productivity and man-power that could be used outside of its own frontiers. Some of the surplus was used outside--to the south, into Central Africa, to the west into North Africa, to the north into Eastern Europe and Western Asia, inaugurating the second phase of Egyptian development. During this second phase Egyptian wealth, population and technology, spilling over its frontiers onto foreign lands, established and maintained relations with foreign territory on a basis that yielded a yearly "tribute," paid by foreigners into the Egyptian treasury. The land of Egypt thus surrounded itself with a cluster of dependencies, converting what had been an independent state or independent states into a functioning empire. The land of Egypt was the nucleus of the Egyptian Empire--center of wealth and power with its associates and its dependencies. The empire was held together by a legal authority using armed force where necessary to assert or preserve its identity and unity. Expansion, the third phase of Egyptian development, involved the export of culture traits and artifacts beyond national frontiers, extending the cultural influence of Egypt into non-Egyptian lands inhabited by Egypt's neighbors. Merchants, tourists, travelers, explorers and military adventurers carried the name and fame of Egypt into other centers of civilization and into the hinterland of barbarism that surrounded the civilizations of that period. Thus the land of Egypt expanded into the Egyptian Empire and the culture of Egypt (its language, its ideas, its artifacts, its institutions) expanded far beyond the boundaries of Egyptian political authority and established Egyptian civilization in parts of Africa, Asia and Europe. The era of Egyptian civilization was divided into two periods by an invasion of the Hyksos, nomadic leaders who moved into Egypt, ruled it for a period and later were expelled and replaced by a new Egyptian dynasty. The fourth period of Egypt's experiment with civilization was that of decline. From a position of political supremacy and cultural ascendancy Egyptian influence weakened politically, economically, ideologically and culturally until the year of the Persian Conquest, 525 B.C., when Egypt became a conquered, occupied, provincial and in some ways a colonial territory.