Persian Spirit, Skill, and Resourcefulness
The basis of the [Persian military and political] system [circa 500 B.C.E.] was the spirit, skill, and resourcefulness of the Persians. An important weapon was the bow, used effectively by both cavalry and infantry. Insofar as possible the Persians avoided close-quarters infantry combat until their foes had been thoroughly disorganized by swarms of foot archers from the front, and the daring onrushes of horse archers against flanks and rear. The Persians were versatile in adapting their methods of warfare to all conditions of terrain. They respected the shock action of the Lydian cavalry lancers, and incorporated this concept into their mounted tactics.
Subject peoples were required to render military service. The garrisons scattered throughout the empire were principally composed of unit from other regions…but always contained a Persian contingent. Imperial expeditionary forces were also multinational. The Persians received a surprisingly high standard of loyalty from these diverse peoples, due largely to their policies of leniency toward the conquered, and of carefully supervised but decentralized administration.
— The Encyclopedia of Military History, p. 22