Notes on the Bible, by Albert Barnes, [1834], at sacred-texts.com
On comparing this whole chapter with Sa2 8:3-13; and 1 Chr. 19 with 1 Chr. 18, it seems not improbable that they are two accounts of one and the same war; the former account Sa2 8:3-13 being inserted out of its chronological order. The numbers slain on both occasions, 42,000 Sa2 8:4-5, 40,000 Sa2 10:18, 700 Sa2 8:4; Sa2 10:18, the seat of war, the mention of the Euphrates, the persons engaged - David, Joab, and Abishai on one side, Hadarezer and the vassal kings on the other - are too similar to make it probable that they belong to two different wars.
The king - In marginal reference. Nahash, king, etc. The interval between the two events, not less than 50 years, and possibly more, is against his being the same as the Nahash of Sa1 11:1-15.
The Ammonites are almost always spoken of as the children of Ammon, from the name of their first ancestor Ben-ammi Gen 19:38.
Hanun - The equivalent of the Carthaginian Hanno, from the same root as the Hebrew, Hananiah, Johanan, Hannah, etc. The same name appears in composition with Baal in Baal-Hanan, an Aramean king Gen 36:38-39.
The history does not record any instance of Nahash's kindness to David, but the enmity of the house of Nahash against Saul may have disposed him favorably toward Saul's enemy David, and if there was any family connection between David's house and Nahash Sa2 17:25 this may have increased the friendship.
The princes ... - Compare Rehoboam's advisers Kg1 12:10-11. It is not improbable that David's severe treatment of Moab Sa2 8:2 was in part the cause of the fear of the Ammonites that a similar treatment was in store for themselves.
In Ch1 19:4, more concisely "shaved." Cutting off a person's beard is regarded by the Arabs as an indignity equal to flogging and branding among ourselves. The loss of their long garments, so essential to Oriental dignity, was no less insulting than that of their beards.
Stank ... - A strong figure for to be odious or detested. Compare the marginal references
The Syrians of Beth-rehob - If identical with the Mesopotamians of Ch1 19:6, Beth-rehob is the same as Rehoboth by the river Gen 36:37. Others think Beth-rehob (Rehob, Sa2 10:8) the same as the Rehob and Beth-rehob of Num 13:21, near Hamath (perhaps the modern ruin of Hunin). If so, Beth-rehob, as well as Tob, must have been a colony of Aram Naharaim (compare the numbers in Ch1 19:7 and here).
Syrians of Zoba - Compare Sa1 14:47 note.
King Maacah - Read the "King of Maacah" Ch1 19:6-7. For the position of Maacah, see Deu 3:14; Jos 12:5. It appears to have been a very small state, since its king only brought a thousand men into the field.
Ish-tob - See the margin. Tob was the district where Jephthah fled when driven out by the Gileadites.
This sufficiently indicates the greatness of the danger to Israel from this formidable league of Ammonites and Syrians.
Came out - From their city, Rabbah Deu 3:11, Deu 3:15 or 20 miles from Medeba, where Ch1 19:7 the Syrian army was encamped. Medeba (modern Madeba) was taken from Sihon Num 21:30, and fell to Reuben Jos 13:9, Jos 13:16; in the reign of Ahaz it seems to have returned to Moab Isa 15:2, and in the time of the Maccabees to the Amorites (1 Macc. 9:36, 37). In church history it was a bishop's see.
In the field - i. e. in the plain below the round rocky hill on which the city stood.
The two armies of the Ammonites and the Syrians were drawn up facing one another; the Ammonites supported by the city Rabbah behind them; the Syrians in great force, with numerous chariots able to manoeuvre in the plain in front of Medeba. If Joab advanced against either, he would have the other in his rear.
For the cities of our God - This rather indicates that the relief of Medeba was one of the immediate objects in view, and consequently that at this time Medeba was still in the possession of the Reubenites. To prevent an Israelite city falling into the hands of a pagan people, and the rites of Moloch being substituted for the worship of Yahweh, was a very urgent motive to valor.
Joab returned - The great strength of Rabbah made it hopeless to take it by assault, and the Syrians were not sufficiently broken Sa2 10:15 to make it safe to undertake a regular siege.
Helam - The place is unknown. Some prefer the translation of the Latin Vulgate: "their host came."
Seven hundred chariots - More probable than the "seven thousand" of Ch1 19:18. The frequent errors in numbers arise from the practice of expressing numerals by letters, with one or more dots or dashes to indicate hundreds, thousands, etc.
Servants to Hadarezer - This gives us an idea of the great power of Hadarezer, and consequently of the strength of Israel in David's victorious reign.