Matthew Poole and continuators on 1 Kings 4:21

21 And Solomon reigned over all kingdoms, from the river unto the land of the Philistines , and unto the border of Egypt : they brought presents, and served Solomon all the days of his life.

Euphrates: for so far David, having conquered the Syrians, extended his Empire, which Solomon also maintained in that extent. And so Gods promise concerning the giving of the whole Land, as far as Euphrates, to the Israelites, was fulfilled. And, if the Israelites had multiplied so much that the Land of Canaan would not suffice them, having Gods grant of all the Land as far as Euphrates, they might have seized upon it whensoever occasion required.

Which is to be understood inclusively; for it is unreasonable to think that the Philistines were not within Solomon's dominion.

Unto the River Sihor, which was the border between Egypt and Canaan, Ios. 13. 3. Compare Gen. 15. 18.

By Tribute, or other ways, as he needed and required.