What factors explain the Persian (Archaemenid) empire's survival for two hundred years?
Until the sixth century BC, they were a people shrouded in mystery. Living in the area east of the Mesopotamian region, the Persians were a disparate group of Indo-European tribes, some nomadic, some settled, that were developing their own culture and religion unique from that of the great cities to their west. Sometimes history is about ideas, and nothing more clearly emphasizes this aspect of history than the sudden eruption of Persians on to the world stage, or at least the world stage as it centered around Mesopotamia. For the sudden rise of Persian power not only over Mesopotamia, but over the entire known world, has its center of gravity in a new set of ideas constellating around a new religion. For the Persians would become the largest and most powerful empire ever known in human history up until that point.
The last known record of the Persians comes from an Assyrian inscription from c. 844 BC that calls them the Parsu (Parsuash, Parsumash) and mentions them in the region of Lake Urmia alongside another group, the Madai (Medes). For the next two centuries, the Persians and Medes were at times tributary to the Assyrians. The region of Parsuash was annexed by Sargon of Assyria around 719 BC. Eventually the Medes came to rule an independent Median Empire, and the Persians were subject to them. The Achaemenids were the first to create a centralized state in Persia, founded by Achaemenes (Hakhamanish), chieftain of the Persians around 700 BC.
When Cyrus "The Great," entered Babylon in 539 B.C. The Persian Empire began. Stretching from the Indus River to all the way to Egypt. This Reign of power is considered a model for the future empires because of their...