Ammoron wrote to Helaman asking to exchange Lamanite prisoners for the city of Antiparah. Note that the date Helaman gives was the end of the “twenty-eighth year of the judges.” Surprisingly, this request for a prisoner exchange occurred somewhat before the famous prisoner-exchange-negotiations between Moroni and Ammoron, which was at the beginning of the twenty-ninth year of the judges. Recall that Helaman was writing a letter to Moroni about what had previously happened, a history. Another brief flashback.
Ammoron was fighting the city of Mulek, and was desperate for more soldiers. Getting back his prisoners would have greatly enhanced his military strength. Here he wanted to exchange them for a city. Helaman refused to trade prisoners for the city, and the Lamanites voluntarily abandoned Antiparah to Helaman without bloodshed. About a year later, Ammoron wanted to exchange prisoners, this time with Moroni, who refused, and promptly rescued the prisoners.