Shared ground
This verse presents a normal wartime rhythm: a customary season arrives “when kings go out to battle,” and Israel conducts a major campaign against Ammon. The text explicitly credits Joab with leading the army, devastating the surrounding Ammonite territory, and then tightening the action around the key target—Rabbah—until it is brought down.
It also explicitly highlights David’s absence from the battlefield. The sentence sets “David stayed in Jerusalem” in contrast to the expected royal pattern it has just named.
Where interpretation differs (only where needed)
Some readers take the note about David staying in Jerusalem as mainly a historical marker—explaining how the campaign was run (through Joab) and setting up the next reported events.
Others think the narrator’s contrast (“when kings go out… but David stayed”) invites a moral evaluation: David’s choice is portrayed as a noteworthy deviation, possibly hinting at a lapse in royal responsibility, even though this verse itself does not state any motive or consequence.
Why the disagreement exists
The verse contains an explicit norm (“time when kings go out”) and an explicit contrast (“but David stayed”), yet it gives no stated reason, emotion, or divine comment. Because Chronicles often reports events selectively and briefly, readers differ on whether this contrast is simply informative or intentionally critical.
What this passage clearly contributes
Textually, the verse establishes (1) the campaign’s timing, (2) Joab’s delegated leadership, (3) the strategy of weakening a city by ravaging its countryside, (4) the siege of Rabbah, (5) David’s remaining in Jerusalem, and (6) the city’s fall in compressed summary.
Theologically (as inference, not direct statement), it supports a picture of David’s kingdom operating through appointed commanders, and it shows that royal presence is normally expected in war—even when the narrative focuses on outcomes rather than inner motives. It also aligns with Chronicles’ broader interest in Jerusalem as the royal center of administration, even while military action happens elsewhere (compare the kingdom-security emphasis around David’s reign in 1 Chronicles 17:11–14).