Mormon 3:6

And there we did place our armies, that we might stop the armies of the Lamanites, that they might not get possession of any of our lands; therefore we did fortify against them with all our force.

We put our armies there. We wanted to stop the Lamanite armies. We did not want them to take our lands. So we used all our strength to build defenses against them.

We positioned our armies there to block the Lamanites from taking any of our territory. We fortified our defenses with everything we had.