Gilead is mine, and Manasseh is mine - That is, My dominion or authority is extended over these regions - Gilead, Manasseh, Ephraim, and Judah. The idea here is substantially the same as in the former verse, that his dominion extended over the country on both sides of the Jordan; or that in the direction of east and west it embraced all that had been promised - “from the great sea to the river Euphrates.” In verse 6, this idea is expressed by selecting two spots or towns as representatives of the whole country - Shechem on the west, and Succoth on the east; in this verse, the same idea is expressed by a reference to the two regions so situated - Gilead and Manasseh on the east, and Ephraim and Judah on the west. Gilead was on the east of the river Jordan, properly embracing the mountainous region south of the river Jabbok, Genesis 31:21-48; Song of Solomon 4:1. The word has sometimes, however, a wider signification, including the whole mountainous tract between the rivers Arnon and Bashan, and thus including the region occupied by the tribes of Gad, Reuben, and Manasseh, Numbers 32:26, Numbers 32:29, Numbers 32:39. Hence, in this place, it is put for the region occupied by the tribes of Reuben and Gad. “Manasseh” refers to the district or region occupied by the half tribe of Manasseh, on the east of the Jordan. These two portions - Gilead and Manasseh - or, Reuben, Gad, and Manasseh - would, therefore, embrace the whole of the land of promise, north and south, on the east of the Jordan. The limits of these regions to the east were properly the banks of the Euphrates; that is, the original promise would embrace this. David had gone to carry the boundaries of his country to those assigned limits 2 Samuel 8:3, and he now says that he had completed that undertaking.
Ephraim also - Ephraim and Judah were the principal tribes on the west of the Jordan, and they would well represent that part of Canaan. The idea is, that the whole of the promised land, east and west, was now under his control. There needed only the territory of Edom, on the south, to complete the conquest, and place the whole of the promised land under his dominion, Psalms 60:8-9.
Is the strength of my head - This means that Ephraim constituted his chief strength, or was that on which he mainly relied. It was that which protected him, as the helmet does the head; that on which his very life in battle depended. This honor is given to the tribe of Ephraim because it was one of the largest tribes, and because it was situated in the very center of the land.
Judah is my lawgiver - This means that the tribe of Judah, by its position, its numbers, and the prominence given to it in the prophecies Genesis 49:8-12, actually gave law to the nation. Its influence was felt in all the institutions of the land. The controlling influence went out from that tribe in the time of David; and its authority in this respect was recognized, perhaps partly in anticipation of what it had been said would be its importance in future times: “The scepter shall not depart from Judah, nor a lawgiver from between his feet until Shiloh come.” Genesis 49:10.