So Daniel prospered during the reign of Darius and the reign of Cyrus the Persian.
The word צלח, tzelech, properly signifies to “pass over,” and the signification is here metaphorical, in the sense of being prosperous. There is no doubt, however, of there being a silent contrast between the kingdom of the Persians and the Chaldean monarchy, that is, to speak more concisely and clearly, between the twofold condition of Daniel.
Darius here studies to make some amends for the dishonour he had done both to God and Daniel, in casting Daniel into the lions' den, by doing honour to both. I. He gives honour to God by a decree published to all nations, by which they are required to fear before him.
Commenting on Daniel 6:25-28
It was in the third year of Cyrus that Daniel's visions (Dan. 10:1-12:13) were given. Daniel "prospered" because of his prophecies (Ezr 1:1-2). This chapter treats of the same subject as the second chapter. But there the four kingdoms, and Messiah's final kingdom, were regarded according to their external political aspect, but here according to the mind of God concerning them, and their moral features.