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
1 Chronicles 20:1
ch1 20:1RABBAH BESIEGED BY JOAB, SPOILED BY DAVID, AND THE PEOPLE TORTURED. (Ch1 20:1-3)
at the time when kings go out to battle--in spring, the usual season in ancient times for entering on a campaign; that is, a year subsequent to the Syrian war.
Joab led forth the power of the army, and wasted the country . . . of Ammon--The former campaign had been disastrous, owing chiefly to the hired auxiliaries of the Ammonites; and as it was necessary, as well as just, that they should be severely chastised for their wanton outrage on the Hebrew ambassadors, Joab ravaged their country and invested their capital, Rabbah. After a protracted siege, Joab took one part of it, the lower town or "city of waters," insulated by the winding course of the Jabbok. Knowing that the fort called "the royal city" would soon fall, he invited the king to come in person, and have the honor of storming it. The knowledge of this fact (mentioned in Sa2 12:26) enables us to reconcile the two statements--"David tarried at Jerusalem" (Ch1 20:1), and "David and all the people returned to Jerusalem" (Ch1 20:3).
1 Chronicles 20:2
ch1 20:2David took the crown of their king . . ., and found it to weigh a talent of gold--equal to one hundred twenty-five pounds. Some think that Malcom, rendered in our version "their king," should be taken as a proper name, Milcom or Molech, the Ammonite idol, which, of course, might bear a heavy weight. But, like many other state crowns of Eastern kings, the crown got at Rabbah was not worn on the head, but suspended by chains of gold above the throne.
precious stones--Hebrew, a "stone," or cluster of precious stones, which was set on David's head.
1 Chronicles 20:3
ch1 20:3cut them with saws, &c.--The Hebrew word, "cut them," is, with the difference of the final letter, the same as that rendered "put them," in the parallel passage of Samuel [Sa2 12:31]; and many consider that putting them to saws, axes, and so forth, means nothing more than that David condemned the inhabitants of Rabbah to hard and penal servitude.
1 Chronicles 20:4
ch1 20:4THREE OVERTHROWS OF THE PHILISTINES AND THREE GIANTS SLAIN. (Ch1 20:4-8)
war at Gezer--or Gob (see Sa2 21:18-22).