Moroni had given orders to keep prisoners for just such an exchange (Alma 52:8). Ammoron now opens that opportunity.
Culture: The ravages of war may be seen in the reasons Mormon gives for Moroni’s joy at this offer. First, he will no longer need to feed the Lamanite prisoners, a telling reminder of food scarcity. Moroni does not begrudge feeding the Lamanites because they are Lamanites, but because Nephites are in need. The depletion of Nephite military strength is also implied in Moroni’s joy at the prospect of bringing back Nephites who might return to the lines.
Chronological: The beginning of the twenty-ninth year would be approximately 65 B.C.