Albert Barnes Commentary 2 Samuel 18:6

"So the people went out into the field against Israel: and the battle was in the forest of Ephraim." — 2 Samuel 18:6 (ASV)

Against Israel - This implies that the revolt was largely that of the ten tribes, Saul’s party, against the kingdom.

The wood of Ephraim - This location would naturally be sought west of the Jordan (as noted in the marginal reference). However, it seems certain that the scene of this battle was east of the Jordan.

Therefore, it is necessary to conclude that some portion of the thick wood of oaks and terebinths, which still runs down to the Jordan on the east side, was called “the wood of Ephraim” for a particular reason. This name could have arisen either because it was a continuation of the great Ephraimite forests on the west, or because of a past event that took place there involving the tribe of Ephraim, such as the slaughter of the Midianites (Judges 7:24–25) or their own slaughter (Judges 12:6).