A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
2 Kings (2 Samuel) 18:1
sa2 18:1DAVID REVIEWING THE ARMIES. (Sa2 18:1-4)
David numbered the people that were with him--The hardy mountaineers of Gilead came in great numbers at the call of their chieftains, so that, although without money to pay any troops, David soon found himself at the head of a considerable army. A pitched battle was now inevitable. But so much depending on the life of the king, he was not allowed to take the field in person; and he therefore divided his forces into three detachments under Joab, Abishai, and Ittai, the commander of the foreign guards.
2 Kings (2 Samuel) 18:5
sa2 18:5GIVES THEM CHARGE OF ABSALOM. (Sa2 18:5-13)
Deal gently for my sake with the young man, even with Absalom--This affecting charge, which the king gave to his generals, proceeded not only from his overwhelming affection for his children, but from his consciousness that this rebellion was the chastisement of his own crimes, Absalom being merely an instrument in the hand of retributive Providence;--and also from his piety, lest the unhappy prince should die with his sins unrepented of.
2 Kings (2 Samuel) 18:6
sa2 18:6wood 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.
2 Kings (2 Samuel) 18:7
sa2 18:7the people of Israel were slain--This designation, together with the immense slaughter mentioned later, shows the large extent to which the people were enlisted in this unhappy civil contest.
2 Kings (2 Samuel) 18:8
sa2 18:8the wood devoured more people than the sword--The thick forest of oaks and terebinths, by obstructing the flight, greatly aided the victors in the pursuit.
2 Kings (2 Samuel) 18:9
sa2 18:9Absalom met the servants of David--or was overtaken. "It is necessary to be continually on one's guard against the branches of trees; and when the hair is worn in large locks floating down the back, as was the case with a young man of the party to which I belonged, any thick boughs interposing in the path might easily dislodge a rider from his seat, and catch hold of his flowing hair" [HARTLEY]. Some, however, think that the sacred historian points not so much to the hair, as to the head of Absalom, which, being caught while running between two branches, was enclosed so firmly that he could not disengage himself from the hold, nor make use of his hands.
the mule that was under him went away--The Orientals, not having saddles as we do, do not sit so firmly on the beasts they ride. Absalom quitting his hold of the bridle, apparently to release himself when caught in the oak, the mule escaped.
2 Kings (2 Samuel) 18:11
sa2 18:11Joab said unto the man that told him, . . . I would have given thee ten shekels of silver, and a girdle--that is, would have raised him from the ranks to the status of a commissioned officer. Besides a sum of money, a girdle, curiously and richly wrought, was among the ancient Hebrews a mark of honor, and sometimes bestowed as a reward of military merit. This soldier, however, who may be taken as a fair sample of David's faithful subjects, had so great a respect for the king's wishes, that no prospect of reward would have tempted him to lay violent hands on Absalom. But Joab's stern sense of public duty, which satisfied him that there could be neither safety to the king, nor peace to the kingdom, nor security to him and other loyal subjects, so long as that turbulent prince lived, overcame his sensibilities, and looking upon the charge given to the generals as more befitting a parent than a prince, he ventured to disobey it.
2 Kings (2 Samuel) 18:14
sa2 18:14HE IS SLAIN BY JOAB. (2Sa. 18:14-32)
he took three darts . . . and thrust them through the heart of Absalom--The deed, partially done by Joab, was completed by his bodyguard. Being a violation of the expressed wish, as well as of all the fond paternal feelings of David, it must have been deeply offensive to the king, nor was it ever forgotten (Kg1 2:5); and yet there is the strongest reason for believing that Joab, in doing it, was actuated by a sincere regard to the interests of David, both as a man and a monarch.
2 Kings (2 Samuel) 18:16
sa2 18:16Joab blew the trumpet, . . . and held back the people--Knowing that by the death of the usurper there was no occasion for further bloodshed, he put an end to the pursuit and thereby evinced the temperate policy of his conduct. However harsh and unfeeling to the king Joab may appear, there can be no doubt that he acted the part of a wise statesman in regarding the peace and welfare of the kingdom more than his master's private inclinations, which were opposed to strict justice as well as his own interests. Absalom deserved to die by the divine law (Deu 21:18, Deu 21:21), as well as being an enemy to his king and country; and no time was more fitting than when he met that death in open battle.
2 Kings (2 Samuel) 18:17
sa2 18:17they took Absalom, and cast him into a great pit . . . and laid a very great heap of stones upon him--The people of the East indicate their detestation of the memory of an infamous person by throwing stones at the place where he is buried. The heap is increased by the gradual accumulation of stones which passers-by add to it.
2 Kings (2 Samuel) 18:18
sa2 18:18Absalom in his lifetime had reared up for himself a pillar--literally, "hand." In the valley of Jehoshaphat, on the east of Jerusalem, is a tomb or cenotaph, said to be this "pillar" or monument: it is twenty-four feet square, dome-topped, and reaches forty feet in height. This may occupy the spot, but cannot itself be the work of Absalom, as it evidently bears the style of a later architecture.
2 Kings (2 Samuel) 18:19
sa2 18:19Then said Ahimaaz . . . Let me . . . run and bear the king tidings--The reasons why Joab declined to accept Ahimaaz' offer to bear intelligence of the victory to David, and afterwards let him go along with another, are variously stated by commentators--but they are of no importance. Yet the alacrity of the messengers, as well as the eager excitement of the expectants, is graphically described.
2 Kings (2 Samuel) 18:23
sa2 18:23by the way of the plain--or ciccar, "circle." This word is only used elsewhere in connection with the valley of the Jordan. It is possible that there may have been a place or region so called on the tablelands of Gilead, as the Septuagint seems to indicate. Or Mahanaim may have been so situated, with the regard to the battlefield, as to be more easily accessible by a descent to the plain of the Jordan, than over the hills themselves. Or the word may signify (as EWALD explains) a manner of quick running [STANLEY].
2 Kings (2 Samuel) 18:24
sa2 18:24David sat between the two gates--that is, in the tower-house on the wall that overhung the gate of Mahanaim. Near it was a watchtower, on which a sentinel was posted, as in times of war, to notify every occurrence. The delicacy of Ahimaaz' communication was made up by the unmistakable plainness of Cushi's. The death of Absalom was a heavy trial, and it is impossible not to sympathize with the outburst of feeling by which David showed that all thoughts of the victory he had won as a king were completely sunk in the painful loss he had sustained as a father. The extraordinary ardor and strength of his affection for this worthless son break out in the redundancy and vehemence of his mournful ejaculations.