Most curious in verse 12 is the "therefore." The word "therefore" appears to be the logical conclusion of the information in verse 11.
In verse 11 the people of Amlici call themselves Amlicites, and therefore the Nephites understood the military intent of the Amlicites and armed themselves for defense. It is somewhat unusual that the naming of the people would trigger this reaction when the separation of the people to a new community and the naming of a king contrary to the voice of the Nephite people did not.
It is possible that this simply emphasizes the power of a name among ancient peoples. Just as Benjamin wanted a new people by giving them a new name, so do the Amlicites declare themselves to be something different by proclaiming a new name. The separation to a new community was likely the way any of the new communities dependent upon Zarahemla had been created. Unlike those other new communities (presumably) the Amlicites broke not only physically, but conceptually from the Nephites. That more complete change was heralded by a change in name.
It is very possible that there were among these Amlicites kin of the Nephites of the land of Zarahemla. As kin, they would be expected to behave in certain ways, and internal military action would clearly have been prohibited to kin. The naming of the new community might effectively break kin ties. This declaration of a new people might establish the new alliances, and thus military action would be conceivable when the formal kin relations had been declared to be severed.
The new people are also named for their king. As has been continually noted, the nature of Mesoamerican kingship brought with it a correlated set of concepts that the Amlicites would have accepted as well. The naming of the people for a king would perhaps be seen as strengthening the conceptual division between Amlicites and Nephites.
The Nephites would have been quite aware of the economic trappings associated with the Mesoamerican kings, having fought internal contentions over those same tendencies in their society for many years. The acceptance of a king and the king's name may have signaled the full acceptance of Mesoamerican actions, including the wars for tributaries.
For more information on the Nephite weaponry, see the discussion following Enos 1:20.