Jerome : Verse 1. "In the first year of the Darius who was the son of Ahasuerus of the race of the Medes and who reigned over the kingdom of the Chaldeans, in the first year of his reign..." This is the Darius who in cooperation with Cyrus conquered the Chaldeans and Babylonians. We are not to think of that other Darius in the second year of whose reign the Temple was built (as Porphyry supposes in making out a late date for Daniel); nor are we to think of the Darius who was vanquished by Alexander, the king of the Macedonians. He therefore adds the name of his father and also refers to his victory, inasmuch as he was the first of the race of the Medes to overthrow the kingdom of the Chaldeans. He does this to avoid any mistake in the reading which might arise from the similarity of the name. — St. Jerome, Commentary on Daniel, CHAPTER NINE Theodoret of Cyrus : We must distinguish between the reign of Darius son of Ahasuerus and that of Darius the Persian. In this way, the things that are now being read will harmonize with the things that were spoken earlier. He did not simply introduce Darius as Darius the Mede but rather as one "from the race of the Medes." Now, as it is clear, he was not a Mede on both sides of his family, that is, on his father's and mother's side. At the same time he ruled over the kingdom of the Chaldeans, when Belshazzar was killed in the middle of the night by a divine intervention after his godless act. According to my research, this Darius seems to have reigned for a very brief time. — COMMENTARY ON Daniel 9:1-2