Daniel 6:8

Then, having made one decision, they came to the king and said, "Darius, O king, live forever! Your captains and satraps and chiefs and governors have deliberated to set up a royal statute over your kingdom and to confirm an edict, that whoever asks a petition from any God and man for thirty days except from you, O king, shall be cast into the den of the lions. And so now O king, set up the edict and set out a writing because the decree of the Persians and Medes shall not be changed." [Daniel 6:6-8] Then Darius, not perceiving the machination of theirs, purposed a decision, he commended a decree to be written and it to be put forth publicly. For also later Haman wishing to kill Mordecai and his race went to king Artaxerxes and deliberated the same, that by royal decree on the fourteenth of the twelfth month on that one day, all the Jews shall be killed. [Esther 3]

Darius, as he loved quietness and inactivity and found work and business unpleasing, left the care of the empire to his prefects, and for this reason the Scripture adds, he set over the kingdom satraps, that is, 120 governors. And over them he placed three presidents, including Daniel. Then they asked him to set an interdict according to the law of the Medes and Persians. Indeed, among the Persians and the Medes, a law stated that when the king had set a certain rule, it was not permitted to rescind it. So the satraps asked the king for this kind of firm and immutable decree; and they were certain that the king would have not rescinded such a decree in order to save Daniel. It is likely that Daniel was not present in the hall of justice on that day, when the king argued from morning till evening with his satraps in order to save Daniel. If he had been present, he would have immediately confessed that he had prayed [to God], and after the confession of the crime the trial would have not been extended until evening.

Verse 8. "Now therefore, O king, confirm the measure and write the decree so that it may not be altered, according to the custom established by the Medes and Persians." It is perfectly evident, as we have remarked above, that there was only one kingdom of the Medes and Persians both, under the rule of Darius and Cyrus.