Bible Notes Online - Isaiah 18 - ESV
Commentary

v1-3: Isaiah's attention moves to Cush, in Africa, a region reaching far to the west, towards modern Congo, and including the horn of Africa. (There are commentators who regard Cush as indicative of all Africa.) The tall inhabitants, smooth skinned, having a different speech to the Semitic peoples around Israel, a people feared by others. The nation was powerful, able, in the time of Asa, to muster an army of 1 million men (2 Chr 14.9). It had fleets of sailing boats.

v4-6: The Lord looks down from His dwelling place, and pronounces judgment, bringing the nation, once so powerful, into desolation.

v7: The Lord God is over all the nations, bringing them into submission. The people of Cush, in common with all others, will bring their gifts to God, Rev 21.26. Peoples throughout Africa can see here a promise of God's restoration.