Jamieson, Fausset and Brown on Daniel 6:8

8. decree--or, "interdict."

that it be not changed--(Es 1:19; 8:8). This immutability of the king's commands was peculiar to the Medes and Persians: it was due to their regarding him infallible as the representative of Ormuzd; it was not so among the Babylonians.

Medes and Persians--The order of the names is an undesigned mark of genuineness. Cyrus the Persian reigned subordinate to Darius the Mede as to dignity, though exercising more real power. After Darius' death, the order is "the Persians and Medes" (Es 1:14, 19, &c.).