In 559 BCE, a man named Cyrus became the leader of Persia. He was the great-great-grandson of the first Persian king, Achaemenes—whose name is why historians call this the Achaemenid Persian Empire!
Prior to Cyrus’s rule, Persia was a small tributary state to the Median Empire, which happened to be ruled by Cyrus’s grandfather, Astyages. Persia paid the Medes for protection and to maintain a level of independence.
Cyrus came into conflict with his grandfather—for reasons that are unknown—and initiated a rebellion that ultimately succeeded in 550 BCE. Cyrus commemorated his victory over Astyages by building a city on the site of the battle and naming it Pasargadae, after his tribe.
By defeating Astyages, Cyrus took on his role as ruler of what had been the Median Empire. Not everyone who had been paying tribute to Astyages accepted Cyrus as their new ruler, however. In order to solidify his power, Cyrus had to find ways to bring lesser rulers under his control. His success earned Cyrus the title of "Cyrus the Great."Cyrus was a successful military commander, but he also recognized the need to leave the regions that he conquered in good economic order if they were going to provide him with tribute revenues. To achieve this, Cyrus left local rulers in place after conquering a region, and he allowed the local population to continue practicing their preferred religious traditions. These policies ensured that conquered regions continued to function economically and reduced the chance that they would rebel against him.
In ancient Mesopotamia, a common imperial strategy was to relocate conquered populations to new areas in order to break up their political and cultural unity and make them less dangerous to the ruling power. Cyrus reversed this practice by allowing the Jews, who had been relocated by the Babylonians, to return to Israel and establish a tributary state. While this might appear to be an act of generosity, it was probably a calculated move on the part of Cyrus to help ensure Jewish loyalty, and thus a continuation of his general policy of tolerance. Cyrus’s son, Cambyses II, added to the Achaemenid Empire by conquering Egypt. While Cambyses II was away in Egypt, a man pretending to be his brother tried to take control of the empire. Cambyses died in 522 BCE while returning from Egypt to remove this pretender and was succeeded by a general named Darius.
Although Darius had a legitimate claim in that he was distantly related to Cambyses II, several other claimants to the Persian throne challenged Darius. Many regions saw the resulting chaos as an opportunity to rebel against Achaemenid rule.
Darius eventually established himself as the sole ruler of Persia and reconquered the rebellious regions, growing the Achaemenid Empire to its greatest extent. Partly as a response to the initial challenges that he faced, Darius reorganized the empire by dividing it into satrapies, or provinces. For each satrapy, Darius appointed a satrap—a political governor—and a military commander.
The division of military and political power was meant to prevent regional leaders from becoming too powerful. Unlike the system of local control employed by Cyrus, Darius appointed these satraps directly, meaning that their loyalty was to him. (Internet)
