Jamieson, Fausset and Brown on Isaiah 18:3

3. see ye ... hear ye--rather, "ye shall see ... shall hear." Call to the whole earth to be witnesses of what Jehovah ("He") is about to do. He will "lift up an ensign," calling the Assyrian motley hosts together (Is 5:26) on "the mountains" round Jerusalem, to their own destruction. This (the eighteenth chapter) declares the coming overthrow of those armies whose presence is announced in Is 17:12, 13. The same motive, which led Hezekiah to seek aid from Egypt, led him to accept gladly the Ethiopian Tirhakah's aid (Is 36:6; 37:9). Ethiopia, Egypt, and Judea were probably leagued together against the common enemy, 713 B.C. See notes on the twenty-second chapter, where a difference of tone (as referring to a different period) as to Ethiopia is observable. Horsley takes the "ensign" to be the cross, and the "trumpet" the Gospel trumpet, which shall be sounded more loudly in the last days.