One post shall run to meet another, and one messenger to meet another, to show the king of Babylon that his city is taken at {r} one end,

(r) By turning the course of the river one side was made open and the reeds that grew in the water were destroyed which Cyrus did by the counsel of Gobria and Gabatha Belshazzar's captains.

Wesley's Notes

51:31 At one end - Cyrus entered the city at one end, by the channel of the river, which he had drained, and surprized Belshazzar in the midst of his feast.

Jamieson-Fausset-Brown Bible Commentary

31. (See on [1004]Jer 50:24).

One post-One courier after another shall announce the capture of the city. The couriers despatched from the walls, where Cyrus enters, shall "meet" those sent by the king. Their confused running to and fro would result from the sudden panic at the entrance of Cyrus into the city, which he had so long besieged ineffectually; the Babylonians had laughed at his attempts and were feasting at the time without fear.

taken at one end-which was not known for a long time to the king and his courtiers feasting in the middle of the city; so great was its extent that, when the city was already three days in the enemy's hands, the fact was not known in some parts of the city [Aristotle, Politics, 3.2].

Matthew Henry's Concise Commentary

51:1-58 The particulars of this prophecy are dispersed and interwoven, and the same things left and returned to again. Babylon is abundant in treasures, yet neither her waters nor her wealth shall secure her. Destruction comes when they did not think of it. Wherever we are, in the greatest depths, at the greatest distances, we are to remember the Lord our God; and in the times of the greatest fears and hopes, it is most needful to remember the Lord. The feeling excited by Babylon's fall is the same with the New Testament Babylon, Re 18:9,19. The ruin of all who support idolatry, infidelity, and superstition, is needful for the revival of true godliness; and the threatening prophecies of Scripture yield comfort in this view. The great seat of antichristian tyranny, idolatry, and superstition, the persecutor of true Christians, is as certainly doomed to destruction as ancient Babylon. Then will vast multitudes mourn for sin, and seek the Lord. Then will the lost sheep of the house of Israel be brought back to the fold of the good Shepherd, and stray no more. And the exact fulfilment of these ancient prophecies encourages us to faith in all the promises and prophecies of the sacred Scriptures.