Ancient diplomacy was often more bluster than gentleness, and that is clear in this exchange of letters. Moroni had threatened the Lamanites, and Ammoron threatens back. In Ammoron’s case, he brings up personal grudges (the death of Amalickiah, his brother) and adopted ones (the Nephites had usurped power from Laman).
Ammoron declares that his army is sufficiently powerful that Moroni’s best option would be to lay down arms and surrender. Both men understand that this is all bluster, and neither will cease hostilities so easily.