1 Kings 4:21 (ESV)

21 Solomon ruled over all the kingdoms from the Euphrates to the land of the Philistines and to the border of Egypt. They brought tribute and served Solomon all the days of his life.

Solomon’s reign was the high point of the geographical extension of the rule of Israel. The geographical stretch matches the promise that God gave to Abraham in Genesis 15:18. It is to be understood, nevertheless, that Solomon was not technically the king over all this territory, but in the lands outside the nation of Israel itself, Solomon’s authority was demonstrated in the tribute that was brought, while these nations continued to have their own kings. Yet this is not unlike other empires of the ancient world, such as Assyria and Babylon, in which the ruler of the empire had subject kings under him.