Charles Ellicott Commentary 1 Chronicles 20:1

"And it came to pass, at the time of the return of the year, at the time when kings go out [to battle], that Joab led forth the army, and wasted the country of the children of Ammon, and came and besieged Rabbah. But David tarried at Jerusalem. And Joab smote Rabbah, and overthrew it." — 1 Chronicles 20:1 (ASV)

After the year had ended. — In Hebrew, this means at the time of the return of the year, that is, in spring. (See 1 Kings 20:22; 1 Kings 20:26.)

At the time when kings go out. — See 1 Kings 20:16. Military operations were commonly suspended during winter. The Assyrian kings have chronicled their habit of making yearly expeditions of conquest and plunder. It was exceptional for the king to “remain in the country.”

Joab led out the main force of the army. — Samuel gives details: David sent Joab and his servants (possibly the contingents of tributaries, see 1 Chronicles 19:19) and all Israel (that is, the entire national army).

Wasted the country. — An explanation from Samuel: wasted the sons of Ammon.

Rabbah, or Rabbath Ammon, the capital. (See 2 Samuel 11:1; Amos 1:14; Jeremiah 49:2–3.)

But David remained (in Hebrew, was tarrying) at Jerusalem. — While Joab’s campaign was in progress, in 2 Samuel 11:1 this remark prepares the way for the account found there of David’s temptation and fall.

And Joab struck Rabbah, and destroyed it. — This is a brief statement, summarizing the events related in 2 Samuel 11:27. From that passage we learn that, after an assault that doubtless reduced the defenders to the last stage of weakness, Joab sent a message to David at Jerusalem to come and claim the honor of the capture. Our 1 Chronicles 20:2, which abruptly introduces David himself as present at Rabbah, obviously implies a knowledge of the narrative as it is told in Samuel, and would hardly be intelligible without it. Whether the chronicler here and elsewhere borrows directly from Samuel, or from another document depending ultimately on the same original source as Samuel, cannot be decided with certainty.