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) 10:2
sa2 10:2DAVID'S MESSENGERS, SENT TO COMFORT HANUN, ARE DISGRACEFULLY TREATED. (Sa2 10:1-5)
Then said David, I will show kindness unto Hanun the son of Nahash, as his father showed kindness unto me--It is probable that this was the Nahash against whom Saul waged war at Jabesh-gilead (Sa1 11:11). David, on leaving Gath, where his life was exposed to danger, found an asylum with the king of Moab; and as Nahash, king of the Ammonites, was his nearest neighbor, it may be that during the feud between Saul and David, he, through enmity to the former, was kind and hospitable to David.
2 Kings (2 Samuel) 10:3
sa2 10:3the princes of the children of Ammon said unto Hanun--Their suspicion was not warranted either by any overt act or by any cherished design of David: it must have originated in their knowledge of the denunciations of God's law against them (Deu 23:3-6), and of David's policy in steadfastly adhering to it.
2 Kings (2 Samuel) 10:4
sa2 10:4Hanun took David's servants, and shaved off the one half of their beards--From the long flowing dress of the Hebrews and other Orientals, the curtailment of their garments must have given them an aspect of gross indelicacy and ludicrousness. Besides, a knowledge of the extraordinary respect and value which has always been attached, and the gross insult that is implied in any indignity offered, to the beard in the East, will account for the shame which the deputies felt, and the determined spirit of revenge which burst out in all Israel on learning the outrage. Two instances are related in the modern history of Persia, of similar insults by kings of haughty and imperious temper, involving the nation in war; and we need not, therefore, be surprised that David vowed revenge for this wanton and public outrage.
2 Kings (2 Samuel) 10:5
sa2 10:5Tarry at Jericho--or in the neighborhood, after crossing the fords of the Jordan.
2 Kings (2 Samuel) 10:6
sa2 10:6THE AMMONITES OVERCOME. (Sa2 10:6-14)
when the children of Ammon saw that they stank before David--To chastise those insolent and inhospitable Ammonites, who had violated the common law of nations, David sent a large army under the command of Joab, while they, informed of the impending attack, made energetic preparations to repel it by engaging the services of an immense number of Syrian mercenaries.
Beth-rehob--the capital of the low-lying region between Lebanon and Anti-Lebanon.
Zoba--(see on Sa2 8:3).
of king Maacah--His territories lay on the other side of Jordan, near Gilead (Deu 3:14).
Ish-tob--that is, "the men of Tob"--the place of Jephthah's marauding adventures (see also Ch1 19:6; Psa 60:1, title). As the Israelite soldiers poured into the Ammonite territory, that people met them at the frontier town of Medeba (Ch1 19:7-9), the native troops covering the city, while the Syrian mercenaries lay at some distance encamped in the fields. In making the attack, Joab divided his forces into two separate detachments--the one of which, under the command of his brother, Abishai, was to concentrate its attack upon the city, while he himself marched against the overwhelming host of mercenary auxiliaries. It was a just and necessary war that had been forced on Israel, and they could hope for the blessing of God upon their arms. With great judgment the battle opened against the mercenaries, who could not stand against the furious onset of Joab, and not feeling the cause their own, consulted their safety by flight. The Ammonites, who had placed their chief dependence upon a foreign aid, then retreated to entrench themselves within the walls of the town.
2 Kings (2 Samuel) 10:14
sa2 10:14So Joab returned and came to Jerusalem--Probably the season was too far advanced for entering on a siege.
2 Kings (2 Samuel) 10:16
sa2 10:16THE SYRIANS DEFEATED. (Sa2 10:15-19)
Hadarezer sent and brought out the Syrians that were beyond the river--This prince had enjoyed a breathing time after his defeat (Sa2 8:3). But alarmed at the increasing power and greatness of David, as well as being an ally of the Ammonites, he levied a vast army not only in Syria, but in Mesopotamia, to invade the Hebrew kingdom. Shobach, his general, in pursuance of this design, had marched his troops as far as Kelam, a border town of eastern Manasseh, when David, crossing the Jordan by forced marches, suddenly surprised, defeated, and dispersed them. As a result of this great and decisive victory, all the petty kingdoms of Syria submitted and became his tributaries (see on Ch1 19:1).