An empire this big posed some serious logistical challenges. The Persians may have had relatively loyal subjects, after all, but if it took months for messages to reach them, even loyal subjects could make decisions that the kings would disagree with. To help address this issue, Darius undertook a series of major reforms. The Persians continued the Assyrian practice of building highways and setting up supply posts for their messengers. The most important of these highways was called the Royal Road, linking up the empire all the way from western Anatolia to the Persian capital of Susa, just east of the Tigris. A messenger on the Royal Road could cover 1,600 miles in a week on horseback, trading out horses at posts along the way. The Persians standardized laws and issued regular coinage in both silver and gold. The state used several languages to communicate with its subjects, and the government sponsored a major effort to standardize a new, simplified cuneiform alphabet.
As described above, the key to Persian rule was the novel innovation of treating conquered people with a degree of leniency (in stark contrast to the earlier methods of rule employed by the Assyrians and Neo-Babylonians). So long as they were loyal, paid taxes, and sent troops when called, the Persian kings had no problem with letting their subjects practice their own religions, use their own languages, and carry on their own trading practices and customs. For example, it was Cyrus who allowed the exiled Jews to return to Judah from Babylon in the name of a kind of kingly generosity. It seems that the Persian kings felt it very important to maintain an image of beneficence, of linking their power to sympathy for their subjects, rather than trying to terrorize their subjects into submission.
The Persian kings introduced a system of governance that allowed them to gather intelligence and maintain control over such a vast area relatively successfully. The empire was divided into twenty Satrapies (provinces), ruled by officials called Satraps. In each Satrapy, the Satrap was the political governor, advised and supplemented by a military general who reported directly to the king; in this way, the two most powerful leaders in each Satrapy could keep an eye on each other. In addition, roaming officials called the “eyes and ears of the king” traveled around the empire checking that the king’s edicts were being enforced and that conquered people were not being abused, then reporting back to the Persian capitals of Susa and Persepolis (both cities served as royal capitals). Despite that system of political “checks and balances,” the satraps appointed the new king from the royal family when the old one died; sometimes they preferred to appoint weak-willed members of the royal family so that the satraps might enjoy more personal freedom. Likewise, despite the innovations that Darius introduced in organization, the satraps normally operated with a large degree of autonomy.
The kings themselves adopted the title of “King of Kings.” They were happy to acknowledge the authority of the rulers of the lands they had conquered, but required those rulers to in turn acknowledge the Persian king’s overarching supremacy. Many Persian depictions of the kings depicted them receiving tribute from other, lesser kings who had come to Susa or Persepolis in a show of loyalty and support. In this way, the political authority of the empire was tied together by both the formal bureaucratic structure of the satrapies as well as the bonds of loyalty between the King of Kings and his subject rulers.
One final component of the Persian system was relatively modest taxation. In order to keep taxes moderate, the Persian kings only called up armies (of both Persians and conquered peoples) when there was a war; otherwise the only permanent army was the 10,000-strong elite bodyguard of the king that the Greeks called the “Immortals.” When the Persians did go to war, their subjects contributed troops according to their strengths. The Phoenicians formed the navy, the Medes the cavalry, the Mesopotamians the infantry, and so on. This system worked well on long campaigns, but its weakness was that it took up to two years to mobilize the whole empire for war, a serious issue in the conflicts between Persia and Greece in the long run.