Alma 50:11

And thus he cut off all the strong holds of the Lamanites, in the east wilderness; yea, and also on the west, fortifying the line between the Nephites and the Lamanites, between the land of Zarahemla and the land of Nephi; from the West Sea, running by the head of the river Sidon; the Nephites possessing all the land northward; yea, even all the land which was northward of the land Bountiful, according to their pleasure.

This is how he took away all the strong places of the Lamanites in the east wilderness. He also did the same in the west. He made a strong border between the Nephites and the Lamanites. The border ran between Zarahemla and the land of Nephi. It went from the West Sea along the top of the Sidon River. The Nephites owned all the land to the north. They owned all the land north of the land called Bountiful.

This cut off all the Lamanite strongholds in the eastern wilderness. He also fortified the west, creating a defensive line between the Nephites and Lamanites—between Zarahemla and the land of Nephi—from the West Sea along the headwaters of the river Sidon. The Nephites controlled all the land to the north, all the territory north of Bountiful, as far as they wanted.