By now it is no surprise that there are serious wars in the land. The only important issue is that we remember that the definition of Lamanite is by now political and not lineal. That is, Lamanites are the "gentiles" of the Book of Mormon. It is a collective term for anyone who is against the Nephites.
In this case, it is much more likely that these particular Lamanites are not the same ones who had been vexing the City of Nephi. Those Lamanites would have to come 180 miles away from their normal territories to find Zarahemla. It is much more likely that the war comes from other peoples in closer proximity to Zarahemla, perhaps more related to those with whom Zarahemla had been fighting in the years before the arrival of Mosiah and the other Nephites.