Albert Barnes Commentary 2 Samuel 21:15

"And the Philistines had war again with Israel; and David went down, and his servants with him, and fought against the Philistines. And David waxed faint;" — 2 Samuel 21:15 (ASV)

This passage, like the preceding one in 2 Samuel 21:1–14, is clearly a detached and unconnected extract. It was probably taken from a history of David’s wars, apparently the same source that provided the material for 2 Samuel 5, 2 Samuel 8, and 2 Samuel 23:8–39.

There is no direct clue as to when the events described here took place, but it was probably early in David’s reign while he was still young and active, after the war described in 2 Samuel 5. The Book of Chronicles places these Philistine battles immediately after the capture of Rabbah of the Ammonites (1 Chronicles 20:4–8) but omits David’s adventure described in 2 Samuel 21:15–17.