John Calvin 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 ye inhabitants of the world. He shows that this work of God will be so manifestly excellent as to draw the attention not only of the Jews but of all nations.

When he shall lift up an ensign on the mountains, you will see it. These words, which are in the future tense, are rendered by some, according to the custom of Scripture, in the imperative mood; but it is better to view them as denoting what is future.

It is as if he had said that the most distant nations will be witnesses of this destruction, because not only will the ensign be seen by all, but the sound of the trumpets will be heard throughout the whole world. This will plainly show that the war did not originate with men, but with God himself, who will prove himself to be its author by remarkable tokens.

When wars are carried on, everyone sees clearly what is done; but most people ascribe their beginning and end to chance.

On the other hand, Isaiah shows that all these things ought to be ascribed to God, because he will display his power in a new and extraordinary manner. For sometimes he works so as to conceal his hand and to prevent his work from being perceived by people, but sometimes he displays his hand in such a manner that all are constrained to acknowledge it. And that is what the Prophet meant.