Geography: Mormon summarizes the land of Zarahemla’s holdings before this next round of conflict begins. Moroni has cleaned up the borders by strengthening the eastern boundary, thereby reducing the Lamanites’ opportunity to make an easy entrance deep into Nephite lands. The last attack made it all the way to Jershon.
The boundary line ran roughly west-east. The Nephites solidified that front with strongholds along the most likely entrance points. Zarahemla’s southern line extended south to about the “head” or headwaters of the River Sidon.
According to Mormon, the Nephites held all of the land northward, even north of Bountiful, which appears to be the northernmost land of their possession at this time. I read this passage as saying that the Nephites had no rivals to the north and therefore considered that land “theirs” because it was available to them rather than because it was already under their control.