15 Earlier when David was fighting with Edom, Joab the commander of the army, who had gone up to bury the dead, had struck down all the men in Edom.

Read 1 Kings 11:15 Using Other Translations

For it came to pass, when David was in Edom, and Joab the captain of the host was gone up to bury the slain, after he had smitten every male in Edom;
For when David was in Edom, and Joab the commander of the army went up to bury the slain, he struck down every male in Edom
Years before, David had defeated Edom. Joab, his army commander, had stayed to bury some of the Israelite soldiers who had died in battle. While there, they killed every male in Edom.