Albert Barnes Commentary Isaiah 18:3

"All ye inhabitants of the world, and ye dwellers on the earth, when an ensign is lifted up on the mountains, see ye; and when the trumpet is blown, hear ye." — Isaiah 18:3 (ASV)

All you inhabitants of the world—These are to be regarded as the words of the prophet summoning all nations to attend to what was about to occur. Grotius, however, and some others, suppose that they are the words of the Ethiopians. The meaning is that the events predicted here would be so public as to attract the attention of all the world.

When he—Vitringa supposes that this means the Assyrians lifting up a standard on the mountains of Judea. But the better interpretation is the one which refers it to the people of Nubia mustering their forces for war. This interpretation suggests: All nations observe when that people gathers an army, sounds the trumpet for war, and arrays its military forces for battle. See then the judgments that God will inflict on them—their defeat (Isaiah 18:4–7), and their turning to Yahweh and sending an offering to him (Isaiah 18:7).

According to this interpretation, it will refer to the people making preparation for battle. Perhaps it may mean that they were preparing to join the enemies of Judea—quite probably preparing to join the forces of Sennacherib and to invade Judea. For this purpose, the messengers may have been sent to negotiate the terms of alliance with Sennacherib. The object of the prophecy is to assure the Jews that this people, as well as Sennacherib, would be defeated and that they would yet bring an offering to God (Isaiah 18:7).

Lifts up an ensign—A military standard (see the note at Isaiah 5:26).

And when he blows a trumpet—Also a signal for an army to assemble (see the note at Isaiah 13:2).