Charles Ellicott Commentary 2 Samuel 8:3

"David smote also Hadadezer the son of Rehob, king of Zobah, as he went to recover his dominion at the River." — 2 Samuel 8:3 (ASV)

Hadadezer ... king of Zobah. —This name is sometimes (1 Chronicles 18:3, 1 Chronicles 18:5, 1 Chronicles 18:7, and others) spelled “Hadarezer,” the letters d and r being very similar in Hebrew and easily confused; but the form given here is correct, as Hadad was the chief idol of the Syrians. Zobah (called Aram-Zobah in the title of Psalm 60:0) was a kingdom whose position cannot be exactly determined, but it lay northeast of Israel and was formerly governed by petty kings with whom Saul had wars (1 Samuel 14:47). When or by what means it had become united under a single sovereign is unknown, but from 2 Samuel 8:4, along with 2 Samuel 10:6 and 2 Samuel 10:16, it is clear that he was a monarch of considerable power and controlled tribes beyond the Euphrates.

To recover his border.—Literally, to cause his hand to return, a phrase which in itself might mean either to renew his attack, or to re-establish his power. The latter is shown to be the sense here by the expression in 1 Chronicles 18:3, to establish his dominion, and is translated this way in the Septuagint. What happened is more fully explained in 2 Samuel 10:13–19: the Ammonites had obtained the help of the Syrians. When their combined armies were defeated by David, Hadadezer then attempted to summon to his aid the tribes beyond the river (i.e., the Euphrates), but David cut short his plans by another crushing defeat, which reduced them all to subjection. Our Version inserts the word Euphrates on the authority of the margin of the Hebrew, several manuscripts, and all the ancient versions. The river, however, would in any case mean the Euphrates.