What does Isaiah 18:3 mean?
Isaiah has described swift boats sending ambassadors, or messengers, to faraway people. The picture seems to be of the nations of Ethiopia and Egypt seeking to make alliances to undertake the threat of the powerful Assyrian Empire (Isaiah 18:1–2). Now he calls to everyone in the world. Every nation within reach of Assyria was concerned about the threat to their people and to their allies. Many kings felt the need to prepare for war.The prophet says to all those nations to look for a signal "raised on the mountains." Trumpets and horns were common indicators of war. Banners or other large flags were also used. Isaiah's readers would be familiar with such signals. In this case, it will be the Lord sending the signal, calling all the nations to play their part when the time is right.
Isaiah will make the final point that the Lord God of Israel is still in control of the world; that power does not belong to the Assyrians. The Assyrians, while strong, do not have a fraction of the might of God. The Lord is the one managing the threat to all nations. He will call them into action at the key moment.