This verse explains the nature of Zeniff’s guards. He notes that he has “spies out round about the land of Shemlon” and that the purpose is that he “might guard” against the Lamanites. Thus the guarding occurs when the spies notice preparations of war. Only then do the guards return with their information so that the Zeniffites may not be caught by surprise.
The concept that the people would be “destroyed” by the attack of the Lamanites would appear to suggest that this would be a war of extermination. However, that objective is directly opposed to the motive of subjugation that Zeniff ascribed to the Lamanites. It is more likely that it is the political separateness that would be destroyed rather than all of the people. When the subjugation does occur during Limhi’s reign, there is no destruction, only subjugation and exacted tribute. It is also possible that the work “destroy” is hyperbole, and intended to incite the Zeniffites to the defense of their lands.
Geographic: The elevation of Lehi-Nephi and Shilom over Shemlon provided for many excellent spying vantage points.