Joshua 8:15

15.And Joshua and all Israel made as if they were beaten, etc This is another stratagem. By pretending flight they draw off the enemy to a distance, leaving them no retreat afterwards into the city, which was in flames before they suspected that any disaster was to be apprehended in their rear. Hence, while the king of Ai pursues the Israelites as vanquished, the part of the army which lay hid towards Bethel had sufficient time to take the city, and make it too late for the inhabitants to perceive that they were utterly undone. For after they had been already repulsed, and were everywhere slaughtered, they were overwhelmed with despair on beholding the flames of the city, and so completely surrounded that not an individual could escape.

The question here asked by some, as to whether it is lawful to overcome an enemy by wiles and stratagem, originates in gross ignorance. First, it is certain that wars are carried on not merely by striking blows; for those are considered the best commanders who accomplish more by art and counsel than by mere violence; and secondly, the longer any one has served so as to acquire experience, the better soldier he makes. If war, then, is lawful, it is beyond all controversy that the usual methods of conquering may be lawfully employed, provided always that there be no violation of faith once pledged either by truce or in any other way.