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Why does the Lord send an army of an evil nation to destroy one who has fallen from spirituality? Because the Lord does everything for the good of man, in different ways (Moroni 7:24), I believe that even warfare is an attempt to redeem people, on both sides. A fallen nation could remember God and return and repent, while the advancing army could also see their own wicked ways and repent as well. In every instance, the Lord could have sent fire from heaven, but instead sent people.
See also Helaman 8:7, Alma 24:25-27, Isaiah 7:17