Albert Barnes Commentary 2 Samuel 18:8

"For the battle was there spread over the face of all the country; and the forest devoured more people that day than the sword devoured." — 2 Samuel 18:8 (ASV)

The battle was scattered - Probably Absalom’s forces were far more numerous than David’s; but, most likely by Joab’s skillful generalship, the field of battle was such that numbers did not tell, and David’s veteran troops were able to destroy Absalom’s rabble in detail. The wood entangled them, and was perhaps full of pits, precipices, and morasses (2 Samuel 18:17).