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) 20:1
sa2 20:1SHEBA MAKES A PARTY IN ISRAEL. (Sa2 20:1-9)
Sheba . . . a Benjamite--Though nothing is known of this man, he must have been a person of considerable power and influence, before he could have raised so sudden and extensive a sedition. He belonged to the tribe of Benjamin, where the adherents of Saul's dynasty were still numerous; and perceiving the strong disgust of the other tribes with the part assumed by Judah in the restoration, his ill-designing heart resolved to turn it to the overthrow of David's authority in Israel.
every man to his tents--This proverbial expression may have had its foundation in the fact, that many of the Israelite peasantry adhered to the custom of the patriarchs who tilled land, and yet lived in tents, as Syrian peasants often do still. This was the usual watchword of national insurrection, and from the actual temper of the people, it was followed by effects beyond what he probably anticipated.
2 Kings (2 Samuel) 20:2
sa2 20:2from Jordan even to Jerusalem--The quarrel had broken out shortly after the crossing of the Jordan, between Judah and the other tribes, who withdrew; so that Judah was left nearly alone to conduct the king to the metropolis.
2 Kings (2 Samuel) 20:3
sa2 20:3the king took the ten women his concubines--Jewish writers say that the widowed queens of Hebrew monarchs were not allowed to marry again but were obliged to pass the rest of their lives in strict seclusion. David treated his concubines in the same manner after the outrage committed on them by Absalom. They were not divorced, for they were guiltless; but they were no longer publicly recognized as his wives; nor was their confinement to a sequestered life a very heavy doom, in a region where women have never been accustomed to go much abroad.
2 Kings (2 Samuel) 20:4
sa2 20:4Then said the king to Amasa, Assemble me the men of Judah within three days--Amasa is now installed in the command which David had promised him. The revolt of the ten tribes, probably, hastened the public declaration of this appointment, which he hoped would be popular with them, and Amasa was ordered within three days to levy a force from Judah sufficient to put down the insurrection. The appointment was a blunder, and the king soon perceived his error. The specified time passed, but Amasa could not muster the men. Dreading the loss of time, the king gave the commission to Abishai, and not to Joab--a new affront, which, no doubt, wounded the pride of the stern and haughty old general. But he hastened with his attached soldiers to go as second to his brother, determined to take the first opportunity of wreaking his vengeance on his successful rival.
2 Kings (2 Samuel) 20:8
sa2 20:8Amasa went before them--Having collected some forces, he by a rapid march overtook the expedition at Gibeon, and assumed the place of commander; in which capacity, he was saluted, among others, by Joab.
Joab's garment, that he had put on was girded unto him--in the fashion of travellers and soldiers.
a sword . . . and as he went forth it fell out--that is, out of the scabbard. According to JOSEPHUS, he let it drop on purpose as he was accosting Amasa, that stooping, as it were accidentally, to pick it up, he might salute the new general with the naked sword in his hand, without exciting any suspicion of his design.
He went forth--in a ceremonious manner to meet Amasa, now commander-in-chief, in order to seem to render to that officer, whom he considered as usurping his post, a conspicuous honor and homage.
2 Kings (2 Samuel) 20:9
sa2 20:9took Amasa by the beard with the right hand to kiss him--This act, common with two friends on meeting when one of them returns from a journey, indicates respect as well as kindliness, and the performance of it evinced the deep hypocrisy of Joab, who thereby put Amasa off his guard. No wonder, then, that while this act of friendly gratulation after long absence occupied Amasa's attention, he did not perceive the sword that was in Joab's left hand. The action of Joab was indeed a high compliment, but neither suspicious nor unusual and to this compliment, Amasa paying attention and no doubt returning it with suitable politeness, he could little expect the fatal event that Joab's perfidy produced.
2 Kings (2 Samuel) 20:10
sa2 20:10AMASA IS SLAIN. (Sa2 20:10-13)
smote him . . . in the fifth rib--the seat of the liver and bowels, where wounds are mortal.
struck him not again--that is, despatched him at the first blow.
2 Kings (2 Samuel) 20:11
sa2 20:11He that favoureth Joab, and he that is for David, let him go after Joab--It is a striking proof of Joab's unrivalled influence over the army, that with this villainous murder perpetrated before their eyes they unanimously followed him as their leader in pursuit of Sheba. A soldier conjoined his name with David's, and such a magic spell was in the word "Joab," that all the people "went on"--Amasa's men as well as the rest. The conjunction of these two names is very significant. It shows that the one could not afford to do without the other--neither Joab to rebel against David, nor David to get rid of Joab, though hating him.
2 Kings (2 Samuel) 20:14
sa2 20:14JOAB PURSUES SHEBA UNTO ABEL. (Sa2 20:14-15)
he went through all the tribes of Israel unto Abel--beating up for recruits. But there the prompt marches of Joab overtook and hemmed him in by a close siege of the place.
2 Kings (2 Samuel) 20:15
sa2 20:15Abel of Beth-maachah--a verdant place--the addition of "Maachah" betokening that it belonged to the district Maachah, which lay far up the Jordan at the foot of Lebanon.
2 Kings (2 Samuel) 20:16
sa2 20:16A WISE WOMAN SAVES THE CITY BY SHEBA'S HEAD. (Sa2 20:16-22)
Then cried a wise woman--The appeal of this woman, who, like Deborah, was probably a judge or governess of the place, was a strong one.
2 Kings (2 Samuel) 20:18
sa2 20:18They were wont to speak in old time--The translation of the Margin gives a better meaning, which is to this effect: When the people saw thee lay siege to Abel, they said, Surely he will ask if we will have peace, for the law (Deu 20:10) prescribes that he should offer peace to strangers, much more then to Israelitish cities; and if he do this, we shall soon bring things to an amicable agreement, for we are a peaceable people. The answer of Joab brings out the character of that ruthless veteran as a patriot at heart, who, on securing the author of this insurrection, was ready to put a stop to further bloodshed and release the peaceable inhabitants from all molestation.
2 Kings (2 Samuel) 20:23
sa2 20:23DAVID'S GREAT OFFICERS. (Sa2 20:23-26)
Now Joab was over all the host of Israel--David, whatever his private wishes, found that he possessed not the power of removing Joab; so winking at the murder of Amasa, he re-established that officer in his former post of commander-in-chief. The enumeration of David's cabinet is here given to show that the government was re-established in its wonted course.