The Ammonites were of the family of Lot (Genesis 19:36-38). We have seen in 1 Samuel 2:1 that in Saul's day their king was called "Nahash," which means "serpent." It is thought likely that this was a flattering title given to Ammonite kings because the serpent was considered to be the symbol of wisdom. Of course the wisdom of the world is "devilish" (James 3:15): this is not true wisdom, but subtlety. Ammon is the picture of satanically false religion. Its wicked cruelty was rewarded by a crushing defeat by Saul in 1 Samuel 11:11. if the "Nahash" then ruling was killed in that battle, then Nahash, the father of Hanun was likely his son. Yet it may be the same Nahash, who could be cunning enough to outwardly show kindness to David because of David's separation from Saul. At any rate, the kindness of an Ammonite is always deceitful, and David was not wise to seek to encourage friendship with this enemy of God.
David therefore erred in his too gracious desire to return this kindness by sending men with a message of sympathy concerning the death of the father of Hanun (v.2). The princes of Ammon were suspicious, just as people of false religions are suspicious of the grace of the Lord Jesus Christ. False doctrines never give honest credit to the grace of God, but emphasize the legal principle of man's self-righteousness. Its motives are selfish, therefore it suspects the same selfish motives in others. These princes decide that David's men were merely sent as spies (v.3).
They therefore resort to the gross folly of treating David's men with insulting contempt, shaving off half of their beards and cutting off their garments in the middle as far as their hips, sending them away deeply humiliated (v.4). When David heard this he first gave the men a vacation at Jericho until their beards grew. Nothing is said of how David himself responded to this offensive insult that was leveled at him and at Israel, nor are we told of any move that David made with a view to attacking Ammon before Ammon began preparations for battle.
The Ammonites knew perfectly well that David and Israel would greatly resent this insult, and decided to prepare to take the offensive rather than to wait to defend themselves. They were not confident of their own power to defeat Israel, so that they sent to the Syrians to hire a total of 33,000 soldiers to help them. It may be that the Syrians wanted an opportunity to revenge their former defeat by Israel. These armies gathered together against Israel before we read of David's taking any action. The Ammonites came to "the entrance of the gate," but we are not told what city this was. The Syrians were in the field. Thus Israel was faced with a two-front formidable array.
When Joab went out to meet the Ammonites and Syrians, he evidently considered the Syrians a greater threat than the Ammonites, for he chose choice men to go with him against Syria while the remaining soldiers he sent with Abishai to engage the Ammonites (vs.9-10). They each agreed to help the other if the need arose (v.11). Though it seems doubtful that Joab was a born again man, his words. here (v.12) are good. He knew it was important to give God His place in the battle, and that God would work His own will. Applying this in a personal way is a different matter.
When Joab and his men attacked, the Syrians were quickly put to flight (v.13). We are not told at this time how many were killed, but the Ammonites, seeing the Syrians flee, were themselves taken with fear and turned to flee into the city, which appears to be an Ammonite city (v.14). The victory was gained with apparently not too much bloodshed, and Joab and his army returned to Jerusalem.
However, Syria was still not willing to admit total defeat. Hadadezer, the Syrian king of Zobah, of whom we have read in Chapter 8:3-8 as being soundly defeated by David, was evidently thirsting for revenge and mustered a larger army, enlisting Syrians from east of the Euphrates River to supplement his own large company (v.16).
When David received information of this assemblage, he did not wait for the Syrians to cross the Jordan to attack Israel, but gathered all Israel to cross the Jordan going eastward, to meet the enemy before they came near Jerusalem (v.17). This gave them no time to plot any special strategy. The battle was evidently not very prolonged. The Syrians again fled and David's men killed 700 charioteers and 40,000 horsemen, an enormous decimation of an army that had not previously boasted that total number (v.6). The Syrian commander, Shobach, was among those killed. The Ammonites had evidently faded into the background: they are not even mentioned in this battle, though they had started the whole thing.
Hadadezer and the kings under him could do nothing but accept defeat: they made peace with Israel and submitted to their authority, having been taught a serious lesson not to help the Ammonites (v.11).