John Calvin on Isaiah 18:3

3. All ye inhabitants of the world. He shews 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. {Bogus footnote} These words, which are in the future tense, are rendered by some, agreeably to the custom of Scripture, in the imperative mood; {Bogus footnote} 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 beheld by all, but the sound of the trumpets will be heard throughout the whole world. This will plainly shew that the war did not originate with men, but with God himself, who will prove himself to be the author of it by remarkable tokens. When wars are carried on, every one sees clearly what is done; but the greater part of men ascribe the beginning and end of them to chance. On the other hand, Isaiah shews 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 men, but sometimes he displays his hand in such a manner that all are constrained to acknowledge it; and that is what the Prophet meant.