So the people went out into the field against Israel: and the battle was in the wood of Ephraim;

And the people goeth out into the field to meet Israel, and the battle is in a forest of Ephraim;

So the people went out into the field against Israel, and the fight took place in the woods of Ephraim.

And so, the people departed into the field against Israel. And the battle took place in the forest of Ephraim.

Against Israel - Implying that the revolt was in a great measure that of the ten tribes, Saul's party, against the kingdom.
The wood of Ephraim - This would naturally be sought in the west of Jordan (marginal reference). But on the other hand it seems certain that the scene of this battle was on the east of Jordan. It seems therefore inevitable 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 for some reason called "the wood of Ephraim," either because it was a continuation on the east side of the great Ephraimitic forests on the west, or because of some transaction there in which Ephraim had taken part, such as the slaughter of the Midianites Judges 7:24-25, or their own slaughter Judges 12:6.

So the people went out into the field against Israel: and the battle was in the (c) wood of Ephraim;
(c) So called, because the Ephraimites (as some say) fed their cattle beyond Jordan in this wood.

So the people went out into the field against Israel,.... Josephus (e) calls it a great field, with a wood behind it:
and the battle was in the wood of Ephraim; or near it (f) rather; not in a wood in the tribe of Ephraim, which lay on this side Jordan; whereas this battle was fought on the other side Jordan, in the land of Gilead, not far from Mahanaim, where was this wood; and which was so called, either from the slaughter of the Ephraimites here in the times of Jephthah, Judges 12:4; or from the Ephraimites feeding their cattle here and near it; for the Jews say (g), that Joshua gave them a grant to feed their cattle in any wood in any of the tribes of Israel; and lying near Jordan, they used to drive their cattle over to this place, from whence it had its name.
(e) Ut supra, (Antiqu. l. 7. c. 10.) sect. 2. (f) "ad sylvam", Junius & Tremellius; "prope sylvam", Piscator. (g) In Jarchi, Kimchi, & Abarbinel, in loc.

wood of Ephraim--This wood, of course, was on the east of Jordan. Its name was derived, according to some, from the slaughter of the Ephraimites by Jephthah--according to others, from the connection of blood with the trans-jordanic Manasseh.

Battle in the wood of Ephraim, and death of Absalom. - 2-Samuel 18:6, 2-Samuel 18:7. When the people, i.e., David's army, had advanced into the field against Israel (those who followed Absalom), a battle was fought "in the wood of Ephraim," when Israel was smitten by David's warriors and sustained a loss of 20,000 men. The question, where the "wood of Ephraim" was situated, is a disputed one. But both the name and the fact that, according to Joshua 17:15-16, the tribe-land of Ephraim abounded in forests, favour the idea that it was a wood in the inheritance of Ephraim, on this side of the Jordan; and this is in perfect harmony with the statement in 2-Samuel 18:23, that Ahimaaz took the way of the Jordan valley to bring the news of the victory to David, who was staying behind in Mahanaim. Nevertheless the majority of commentators have supposed that the place alluded to was a woody region on the other side of the Jordan, which had received the name of "wood Ephraim" probably after the defeat of the Ephraimites in the time of Jephthah (Judges 12:1-5). The reasons assigned are, first, that according to 2-Samuel 17:26, Absalom had encamped in Gilead, and it is not stated that he had crossed the Jordan again; secondly, that 2-Samuel 18:3 ("that thou succour us out of the city") presupposes that the battle took place in the neighbourhood of Mahanaim (Thenius); and thirdly, that after the victory the army returned to Mahanaim; whereas if the battle had been fought on this side of the Jordan, it would evidently have been much better for it to remain there and occupy Jerusalem (Ewald, Gesch. iii. p. 237). But neither of these reasons is decisive, and there is no force in the other arguments employed by Thenius. There was no necessity for an immediate occupation of Jerusalem by David's victorious army, since all Israel fled to their tents after the fall of Absalom and the defeat of his army (2-Samuel 18:17 and 2-Samuel 19:9); that is to say, such of Absalom's followers as had not fallen in or after the battle, broke up and returned home, and therefore the revolution was at an end. Consequently there was nothing left for David's army to do but to return to its king at Mahanaim, and fetch him back to Jerusalem, and reinstate him in his kingdom. The other two reasons might have some force in them, if the history before us contained a complete account of the whole course of the war. But even Ewald admits that it is restricted to a notice of the principal battle, which completely crushed the rebellion. There can be no doubt, however, that this was preceded, if not by other battles, yet by such military operations as accompany every war. This is clearly indicated in 2-Samuel 18:6, where it is stated that the army advanced into the field against Israel (2-Samuel 18:6), which evidently refers to such an advance on the part of David's army as might compel Absalom to draw back from Gilead across the Jordan, until at length a decisive battle was fought, which ended in the complete destruction of his army and his own death. Ewald observes still further, that "it seems impossible, at any rate so far as the name is concerned, to assume that the wood of Ephraim was on the other side of the Jordan, whilst according to 2-Samuel 18:23, the messenger who reported the victory went from the field of battle towards the Jordan valley in order to get to David." But the way in which Ewald tries to set aside this important point, as bearing upon the conclusion that the battle took place on this side of the Jordan, - namely, by adopting this rendering of 2-Samuel 18:23, "he ran after the manner of Kikkar, running, and therefore overtook Kushi," - is far too unnatural to meet with acceptance. Under all these circumstances, therefore, we decide in favour of the assumption that the wood of Ephraim is to be sought for in the tribe-territory of Ephraim.
The nature of the ground contributed a great deal to the utter defeat of Absalom.