He shall have dominion also from sea to sea - There is probably an allusion here to the promise in Exodus 23:31 : “And I will set thy bounds from the Red Sea even unto the sea of the Philistines, and from the desert unto the river.” This was the original promise in regard to the bounds of the promised land. A promise similar to this occurs also in Genesis 15:18 : “In the same day the Lord made a covenant with Abram, saying, Unto thy seed have I given this land, from the river of Egypt unto the great river, the river Euphrates.” The meaning here is, that what was implied in these ancient promises would be carried out under the reign of the king referred to in the psalm. The “immediate” allusion, therefore, in the phrase “from sea to sea,” may have been from the Red Sea on the East to the Mediterranean on the West; but still the language is susceptible of a more enlarged application, and may mean from one sea to another; that is, embracing all the lands or countries lying between seas and oceans; or, in other words, that the dominion would be universal. Compare the notes at Psalms 2:8.
And from the river ... - The Euphrates. This was emphatically “the river” to the Hebrews - the great river - the greatest river known to them; and this river would be naturally understood as intended by the expression, unless there was something to limit it. Besides, this was expressly designated in the original covenant as the boundary of the promised land. See, as above, Genesis 15:18. The meaning here is, that, taking that river as one of the boundaries, or as a starting point, the dominion would extend from that to the utmost limits of the earth. It would have no other boundary but the limits of the world. The promise, therefore, is, that the dominion would be universal, or would pervade the earth; at once a kingdom of peace, and yet spreading itself all over the world. It is hardly necessary to say that this did not occur under Solomon, and that it could not have been expected that it would occur under him, and especially as it was expected that his reign would be one of peace and not of conquest. It would find its complete fulfillment only under the Messiah.