While the loss of Amalickiah has temporarily disrupted the Lamanite army, it is not a permanent disruption. Amalickiah’s brother Ammoron takes over command. He becomes the king. The first thing we note is that Ammoron controls his army and makes secure those cities that they had already taken.
The succession of Amalickiah’s brother should prompt a question is modern readers familiar with European style monarchies. From the European perspective, we expect that the son of the king will become the next king, even if that son is quite young. That does not happen here, but rather the brother becomes king. Fortunately, this is very comfortable in the Mesoamerican context. In the later Aztec monarchy, there was frequent succession by brothers (see Susan D. Gillespie. The Aztec Kings. University of Arizona Press, 1989, pp. 11-12).