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Before he defeated the Parthians, though, Ardashir I essentially encircled them by conquering everything around Mesopotamia, slowly moving his army from east to west. <ref> Brosius, p. 140</ref> Once the conquest was complete, the king had to ensure a smooth military and administrative transition from Parthian to Sasanian rule.
Although influenced by the Parthian army in its reliance on cavalry, Ardashir I transformed the powerful Sasanian army into a well-organized fighting force of up to 80,000 men. Companies of 100 men were the numerically smallest level of organization. Ten companies comprised a regiment and ten regiments formed a division. <ref>Brosius, p. 186</ref> Essentially there was great continuity from the Parthian period in the Sasanian army’s reliance on cavalry, but the structure was much more complex.