C. F. Keil and Franz Delitzsch on Joshua 8:2

Jos 8:2

Joshua was to do the same to Ai and her king as he had already done to Jericho and her king, except that in this case the conquerors were to be allowed to appropriate the booty and the cattle to themselves. In order to conquer the town, he was to lay an ambush behind it.The much agitated question, whether it could be worthy of God to employ stratagem in war, to which different replies have been given, has been answered quite correctly by Calvin. “Surely,” he says, “wars are not carried on by striking alone; but they are considered the best generals who succeed through art and counsel more than by force ... . Therefore, if war is lawful at all, it is beyond all controversy that the way is perfectly clear for the use of the customary arts of warfare, provided there is no breach of faith in the violation of treaty or truce, or in any other way.” ארב, a collective noun, signifying the persons concealed in ambush; מארב (Jos 8:9), the place of ambush. “Behind it,” i.e., on the west of the town.