2 Sam 17:25 says "Absalom had appointed Amasa over the army in place of Joab." Why wouldn't David have taken that army with him when he fled Absalom and why would they immediately submit to Absalom as their leader. If an army isn't more disciplined than that, it's not an army, it's just a well armed mob.
As general of Absalom's army, Amasa is an enemy of David. Why would David, (2 Sam 19:13) later appoint him as his general. Replacing Joab is understandable, but why one who previously, so wrongly opposed him.
The answer here is simpler than the previous question.
In 2 Sam 15:13-20 we have a record of who went with David when he escaped Jerusalem. See especially V18:
and all his servants marched past him—all the Cherethites and Pelethites, and six hundred Gittites who had followed him from Gath.
Thus, we know that the following people departed with David:
- The household servants
- Some of the King's family
- His personal body-guards
- David's personal regiments of the Gittites, Cherethites, Pelethites, etc.
- David's senior commanders and advisers such as Joab. This would have been more that 1000 men and possibly a few thousand (2 Sam 18:1). These were the most elite, experienced and bravest of the army.
The main bulk of the army stayed behind because it was not all in Jerusalem but stationed in various places all over Israel. There would have been regiments "from Dan to Beersheba" (2 Sam 17:11). Only the very best, most loyal and senior parts of the army were in Jerusalem.
The description of the battle in 2 Sam 18:1-8 is not specific about the numbers but David placed the command of the elite troops under three experienced commanders for the battle, namely, (2 Sam 18:2) -
He sent out the troops, a third under Joab, a third under Joab’s brother Abishai son of Zeruiah, and a third under Ittai the Gittite.
The rest of David's Army, was placed under the command of Amasa (2 Sam 17:25) and Absalom, numbered in the hundreds of thousands (see 2 Sam 24). It sheer size made it unmanageable, unwieldy and unresponsive. The defeat was decisive.
However, when David made his triumphal return to Jerusalem, he did something characteristic of David to heal wounds - he replaced Joab as head of the army with Amasa (2 Sam 19:13). The purpose is obvious - Ellicott suggests several reasons for the replacement.
(13) Say ye to Amasa.—Amasa, like Joab, was David’s nephew, although possibly his mother may have been only half-sister to David. In this offer of the command-in-chief to the rebel general, David adopted a bold, but a rash and unjust policy. Amasa should have been punished, not rewarded for his treason. He had given no evidence of loyalty, nor was there proof that he would be trustworthy. Moreover, this appointment would be sure to provoke the jealousy and hostility of Joab. But David had long been restless under the overbearing influence of Joab (see 2Samuel 19:22; 2Samuel 16:10; 2Samuel 3:39), and now since he had murdered Absalom, was determined to be rid of him. He therefore took advantage of the opportunity by this means to win over to himself what remained of the military organisation of Absalom.
Benson reaches a similar conclusion:
- Say to Amasa, Art thou not of my bone, &c. — That is, nearly related to me, being my sister’s son. God do so to me, and more also, &c. — He solemnly promises to prefer him to the highest command in the kingdom; for he now thought it a fit time to depress Joab, who was grown insufferably insolent and imperious, and who, through his credit with the army, had protected himself in the commission of the greatest crimes. He had slain Abner most perfidiously in cold blood, and killed Absalom contrary to the king’s express command, and now lately had insulted him in his sorrow. Having, therefore, now an opportunity of transferring the command to another person, who had as great an interest in the people’s favour as Joab, he gladly embraces it, that so he might both chastise Joab for his faults, and rescue himself from the bondage in which that general had hitherto held him. Some, however, have thought that, considering Joab’s very faithful services to David in all the changes of his fortune, and that his violent measures proceeded in part from a regard to him, as judging them necessary for his safety and tranquillity, David’s conduct in this instance, in making Amasa captain of the host in Joab’s room, is not an amiable trait in his character, and was not a prudent step at this time, especially considering Joab’s violent temper: and, it must be acknowledged, it brought on the murder of Amasa.
I can't find much identification of Pelethites or Kerethites, however when I looked at 18:1 as you suggested, it speaks of "thousands" which tells me at least 2000. Thank you.
However I'm still not totally convinced about Amasa. I understand David's problems with Joab, but to trust one of his own family who had betrayed him doesn't sound wise.
Regarding Joab's insolence, David needed someone who would speak without reservation. Joab's scolding for David's behavior before his troops after Absalom's death, victory in their eyes, probably prevented problems wit the men.