Darius I, the Great, was the third king after Cyrus II, the Great, and reigned in Persia for the period 522-486BC. During his entire reign, the governor of the region beyond the River, and Shethar-Boznai, and his companions, the Persians who were in the region beyond the River, were against the re-building of the Temple of God in Jerusalem. They bribed agents to work against the them and to frustrate their plans.
In 520BC the governor’s letter of complaint went to his superior, the satrap who had charge of Babylon and the country west of the Euphrates, no records of the decree of Cyrus II instructing the re-building were found among the archives kept at Babylon. However, a further search found the memorandum at Ecbatana, where Cyrus had resided. Darius I confirmed this decree and ordered his officials to help the Jews.
Darius I reorganized the satrapies. His system of military commanders, introduction of coinage, legal and postal systems lasted as long as the empire of Persia. These facilities and the autonomy allowed to the subject people contributed to the stability of the empire. Under such a system the small community of Jews in Judah (only 42,360 people returned to Judah in response to the Persian king’s decree) was sustained. Jewish officers were appointed to govern them.
We can see how God had preserved the people of Israel in history. Indeed, during the entire period of re-building of the Temple and later the wall, the enemies of the Jews were harassing and threatening them. But God had raised powerful foreign kings to grant the people of God justice, favor and protection. All these fulfilled the promises of God spoken through the prophets of God long before they actually happened.
Indeed, by granting favor to the people of Israel, the Persian kings too continued to sustain their power and considerable prosperity. The Persian Empire which was established by Cyrus the Great lasted from 559BC to 651AD (interrupted by Moslem conquest). The main religion of ancient Persia was Zoroastrianism, but after the seventh century, it was replaced by Islam.