The prophet is describing the arrival of the army. There is an uproar as many people are coming together. Cyrus’s armies were made up of mixed peoples from many nations. Cyrus had conquered different states of Persia one after the other, enlarging his army step by step. As the Lord once brought the Assyrians against Judah, he is now mustering the Medes against Babylon.
4 The sound of a tumult is on the mountains as of a great multitude! The sound of an uproar of kingdoms, of nations gathering together! The LORD of hosts is mustering a host for battle.