Adam Clarke on 1 Kings 4:21

Solomon reigned over all kingdoms - The meaning of this verse appears to be, that Solomon reigned over all the provinces from the river Euphrates to the land of the Philistines, even to the frontiers of Egypt. The Euphrates was on the east of Solomon's dominions; the Philistines were westward on the Mediterranean sea; and Egypt was on the south. Solomon had, therefore, as tributaries, the kingdoms of Syria, Damascus, Moab, and Ammon, which lay between the Euphrates and the Mediterranean. See Calmet. Thus he appears to have possessed all the land that God covenanted with Abraham to give to his posterity.