Albert Barnes 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 - This is the correct form of the name, not Hadarezer. It is the proper spelling, as seen in other names like Benhadad and Hadad (1 Kings 15:18 and following; 1 Kings 11:14 and following). Hadad was the chief deity, or sun-god, of the Syrians.

To recover his border - This literally means "to cause his hand to return." The phrase is used at times in a literal sense (for example, Exodus 4:7; 1 Kings 13:4; Proverbs 19:24) and at other times figuratively (Isaiah 1:25; Isaiah 14:27; Amos 1:8; Psalms 74:11).

The precise meaning of the metaphor must be determined by the context in each case. If, as is most likely, this verse relates to the events detailed more fully in 2 Samuel 10:15–19, the phrase here means that Hadadezer went to renew his attack against Israel, or to gather his forces against them, at the Euphrates River.