9 But let us see what follows; “And Saul said, Let us go down after the strangers, and spoil them. And the priest said, Let us draw near hither unto God.” For in old times God led forth the people to battle; and without His consent no one dared to engage in the fight, and war was with them a matter of religion. For not from weakness of body, but from their sins they were conquered, whenever they were conquered; and not by might and courage, but by favour from above they prevailed, whenever they did prevail.
Victory and defeat were also to them a means of training, and a school of virtue. And not to them only, but to their adversaries; for this was made evident to them too, that the fate of battle with the Jews was decided not by the nature of their arms, but by the life and good works of the warriors. The Midianites at least perceiving this, and knowing that people to be invincible, and that to have attacked them with arms and engines of war would have been fruitless, and that it was only possible to conquer them by sin, having decked out handsome virgins, and set them in the array, excited the soldiers to lasciviousness, endeavouring by means of fornication to deprive them of God's assistance; which accordingly happened.
For when they had fallen into sin, they became an easy prey to all; and those whom weapons, and horses, and soldiers, and so many engines availed not to capture, sin by its nature delivered over bound to their enemies. Shields, and spears, and darts were all alike found useless; but beauty of visage and wantonness of soul overpowered these brave men.
Source: Homilies on the Statues (New Advent)