On the Life of Moses: Books 1-3 (1.306-1.310)

Therefore Moses did not think fit to carry on war against him with his whole army, knowing that superfluous numbers are apt to meet with disaster in consequence of those very numbers; and also, at the same time, thinking it useful to have stations of reserve, to be assistants to those of their allies who appeared likely to fail; but he selected a thousand picked men of the youth of the nation, selected man by man, out of each tribe, twelve thousand in all, for that was the number of the tribes, and he appointed Phinehas to be the commander in the war, as he had already given proof of the happy daring which becomes a general; and after he had offered up sacrifices of good omen, he sent forth his warriors, and encouraged them in the following words:—

"The present contest is not one for dominion or sovereignty, nor is it waged for the sake of acquiring the property of others, though these are the objects for which alone, or almost invariably, wars take place; but this war is undertaken in the cause of piety and holiness, from which the enemy has alienated our relations and friends, being the causes of bitter destruction to those who have been brought under their yoke.

It is therefore absurd for us to be the slayers of our own countrymen, for having offended against the law, and to spare our enemies, who have violated it in a much worse degree, and to slay, with every circumstance of violence, those who were only learning and beginning to sin, but to leave those who taught them to do so unpunished, who are, in reality, the guilty causes of all that has taken place, and of all the evils which our countrymen have either done or suffered."

Therefore being nerved by these exhortations, and being kindled and filled with noble courage which was indeed in their souls already, they went forth to that contest with invincible spirit as to a certain victory; and when they engaged with the enemy, they displayed such incredible vigour and courage that they slew all their enemies, and returned themselves unhurt, every one of them, not one of their number having been slain or even wounded.

Any one who did not know what had taken place, might have supposed, when he saw them returning, that they were coming in, not from war and from a pitched battle, but rather from a display and field-day of exercise under arms, such as often take place in time of peace; and these field-days are days of exercise and practise, while the men train themselves among friends to attack their enemies.