Mormon’s intent is very clear in this verse, even though his history is probably not as clear as his statement. What Mormon wants to point out is that the righteousness of the people leads to peace and prosperity, and their departure from righteousness leads to war and destruction. That black and white dichotomy is echoed in his description of the effect of these contentions on the Nephite people. He suggests that the individual righteous are “delivered at all times” and it is only the wicked who “have been consigned to bondage, or to perish by the sword…”
Of course it is quite unlikely that this clean break between the preservation of the righteous and the destruction of the wicked ever occurred. Mormon’s intent is not for historical accuracy, but religious accuracy.