Alma 43:26 - 43:30
Alma 43: 26 And he caused that all the people in that quarter of the land should gather themselves together to battle against the Lamanites, to defend their lands and their country, their rights and their liberties; therefore they were prepared against the time of the coming of the Lamanites.
Upon arriving in Manti, Moroni organized the people into an army that was ready to take on the Lamanite army when they arrived. He motivated them with the idea that they were going to be defending their lands, families, and liberties because if the Lamanites conquered them they would lose their liberties if not their lives.
There must have been a lot of time between when Moroni found out where the Lamanite army was heading and when they arrived. Either that or Moroni was skilled in getting defenses in place with a great deal of speed.
Alma 43: 27 And it came to pass that Moroni caused that his army should be secreted in the valley which was near the bank of the river Sidon, which was on the west of the river Sidon in the wilderness.
In the meantime, Moroni stationed his forces in the valley on the west side of the Sidon River next to the wilderness so it would not be spotted by the Lamanites when they emerged from the wilderness.
Alma 43: 28 And Moroni placed spies round about, that he might know when the camp of the Lamanites should come.
Moroni also had spies station about so he could keep tracked of the Lamanites and their progress towards Manti so he would not be surprised by them.
Alma 43: 29 And 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;
Moroni was working on the assumption that the Lamanite intentions were to either kill those Nephites that apposed them or force them to be subject to them and the kingdom they would certainly put in place if they were able to conquer.
Alma 43: 30 And 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.
Moroni was operating under this certainty in his mind as to the intentions of the Lamanite commanders. Moroni therefore, thought it not unfair to keep tabs on the progress of the Lamanite armies and use any strategy to defeat them if they attacked. As a result of the reports of the spies, Moroni was able to create a plan of defense as the Lamanites approached.
It is strange that Moroni was worried about what was fair and what was not. All’s fair in love and war is a popular saying, but Moroni was concerned that he was not lowering himself to the level of the Lamanites who could care less about what was fair or not. He knew they would kill anyone who resisted them, whether it be women and children, it did not matter to them. The very survival of the Nephite civilization depended on Moroni at this time.