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.

He also knew that the Nephites only wanted to keep their lands, their freedom, and their church. So he thought it was not wrong to defend them by using a smart plan. His spies found out which way the Lamanites were going to go.

And he knew the Nephites only wanted to preserve their land, their freedom, and their faith. So he saw nothing wrong with defending them through strategy. His spies showed him which route the Lamanites would take.