Daniel 9:1 (ESV)

1 In the first year of Darius the son of Ahasuerus, by descent a Mede, who was made king over the realm of the Chaldeans

The first year of King Darius has arrived. The Babylonian Empire has been completely defeated by the Medes and Persians. It is the same year that Daniel was thrown into the lion’s den. It is striking what we read here about King Darius. Literally it says that he was made king. This seems to indicate that there was someone above him who gave him this high post in the kingdom of the Medes and Persians. It is also notable that he was made king over the kingdom of the Chaldeans. It seems that Darius was not king over the entire empire of the Medes and Persians but only over the area where the Babylonians themselves lived. At this time Cyrus was king over the entire empire and Darius, under Cyrus, was therefore appointed over the most important part of the empire. This also explains why Darius does not appear in the list of kings of the kingdom of the Medes and Persians in extra-biblical material.