In the spring, at the time when kings march out to war, Joab led out the army and ravaged the land of the Ammonites. He came to Rabbah and besieged it, but David remained in Jerusalem. And Joab attacked Rabbah and demolished it.
How the army of the Ammonites and their allies was routed in the field we read in the foregoing chapters. Here we have the destruction of Rabbah, the metropolis of their kingdom (Ch1 20:1), the putting of their king's crown upon David's head (Ch1 20:2), and the great severity that was used towards the people, Ch1 20:3. Of this we had a more full account in 2 Sa.
Commenting on 1 Chronicles 20:1-3
See Gill on Ch1 20:1, Sa2 11:1, Ch1 20:2, Sa2 12:30, Sa2 21:15, and Ch1 21:1. Next: 1 Chronicles Chapter 21
And it came to pass, that after the year was expired, at the time that kings go out [to battle], Joab led forth the power of 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 destroyed it. (a) Which was the chief city of the Ammonites.