Ammoron had been part of the expedition that captured the cities in the Nephite northeastern territories. In addition to cities, they would have also controlled the remaining population of those cities who had not been combatants, likely older men, women and children. These were still left in the land and continued to do what they had done before, supply food and other support to the army. The difference, of course, was that this was now a foreign army and the effort was conscripted rather than voluntary and patriotic.
Because Ammoron had been in the Lamanites’ first attack, he knew that the eastern garrisons in the formerly Nephite cities were well protected. Therefore, he returned to Lamanite lands, undoubtedly with some protective force that could be spared from the defense of the eastern lands. Before returning, he certainly added even more troops. He returns to create a second front that attacked the Nephite western lands.