David brought out the people who were there and put them to work with saws, iron picks, and axes. And he did the same to all the Ammonite cities. Then David and all his troops returned to Jerusalem.
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
cut 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.
He brought out the people - See this transaction particularly explained in the notes on the parallel places, Sa2 12:30-31 (note).