Albert Barnes Commentary 1 Chronicles 20

"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)

"And it came to pass after this, that there arose war at Gezer with the Philistines: then Sibbecai the Hushathite slew Sippai, of the sons of the giant; and they were subdued. And there was again war with the Philistines; and Elhanan the son of Jair slew Lahmi the brother of Goliath the Gittite, the staff of whose spear was like a weaver`s beam." — 1 Chronicles 20:4-5 (ASV)