Alma 35:13

And the people of Ammon departed out of the land of Jershon, and came over into the land of Melek, and gave place in the land of Jershon for the armies of the Nephites, that they might contend with the armies of the Lamanites, and the armies of the Zoramites; and thus commenced a war betwixt the Lamanites and the Nephites, in the eighteenth year of the reign of the Judges; and an account shall be given of their wars hereafter.

The people of Ammon left the land of Jershon and went to the land of Melek. They gave the land of Jershon to the armies of the Nephites so they could fight the armies of the Lamanites and the Zoramites. This is how the war between the Lamanites and the Nephites started in the eighteenth year of the judges. Their wars will be told about later.

The people of Ammon left Jershon and moved to the land of Melek, making room for the Nephite armies to use Jershon as a base against the Lamanite and Zoramite forces. And so in the eighteenth year of the judges, war broke out between the Lamanites and the Nephites. The story of those battles comes later.