Alma 16:2

for behold, the armies of the Lamanites had come in on the wilderness side, into the borders of the land, even into the city of Ammonihah, and began to slay the people, and to destroy the city.

The Lamanite armies had come through the wilderness into the land. They came into the city of Ammonihah and started to kill the people and destroy the city.

Lamanite armies had crossed through the wilderness into Nephite territory, straight into the city of Ammonihah. They began slaughtering the people and destroying the city.