What Nephi describes is not a full war, but rather skirmishes. The nature of the conflict is similar to guerilla warfare, where there is a skirmish, and then a retreat. The Gadiantons are established in strongholds in the wilderness and mountains of their own lands, and use those lands as a base to raid Nephite holdings.
The Nephites meet them in battle and fight them back, but they do not fight until they are defeated or overly depleted, preferring instead to retreat to the wilderness and mountain strongholds, which are strong enough that the Nephites cannot conquer them.