15 Then these men assembled unto the king, and said unto the king, Know, O king, that the law of the Medes and Persians is, That no decree nor statute which the king establisheth may be changed.
Verse 15. "But those men, understanding the king's intent, said to him: 'Be it known to thee, O king, that no law of the Medes and Persians, nor any decree which the king has enacted, is capable of alteration.'" Just as the king understood that the princes were making their accusation out of motives of envy, so also they for their part understood what the king's purpose was, namely that he wished to rescue Daniel from imminent death. And so they allege that according to the law of the Medes and Persians, the commands of a king cannot be nullified.
You do not have the authority to overturn what you have prescribed, the laws of Medes and Persians ordering that the laws passed by the previous king be fulfilled. Under pressure both from the compelling logic of the words and from the number of the accusers, the king surrendered blessed Daniel to the lions.
[AD 420] Jerome on Daniel 6:15