Charles Ellicott 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)

When he lifteth up an ensign ... —Both clauses are better taken as indefinite, when an ensign is set up ... when a trumpet is sounded. The prophet calls on all nations (Ethiopia being specially included) to watch for the signal that shall be given, distinct as the beacon-fire on the hill, or the alarm of the trumpet, to proclaim the downfall of Assyria.