The Persian Empires: Political Structure, Society, & Zorastrianism

How did the satrap system allow the Persians to organize and administer such broad and disparate geographic territories and people?  In what way was the system meant to prevent rebellions against imperial rule? How did leaders like Darius exploit Zoroastrianism as a political tool?  (750-900 words, Times New Roman font, double-spaced, one-inch margins).