Alma 43:29-31

29And now, as Moroni knew the intention of the Lamanites, that it was their intention to destroy their brethren, or to subject them and bring them into bondage that they might establish a kingdom unto themselves over all the land;

30And he also knowing that it was the only desire of the Nephites to preserve their lands, and their liberty, and their church, therefore he thought it no sin that he should defend them by stratagem; therefore, he found by his spies which course the Lamanites were to take.

31Therefore, he divided his army and brought a part over into the valley, and concealed them on the east, and on the south of the hill Riplah;