31:1-6 All who, without commission from God, dare to execute private revenge, and who, from ambition, covetousness, or resentment, wage war and desolate kingdoms, must one day answer for it. But if God, instead of sending an earthquake, a pestilence, or a famine, be pleased to authorize and command any people to avenge his cause, such a commission surely is just and right. The Israelites could show such a commission, though no persons now can do so. Their wars were begun and carried on expressly by Divine direction, and they were enabled to conquer by miracles. Unless it can be proved that the wicked Canaanites did not deserve their doom, objectors only prove their dislike to God, and their love to his enemies. Man makes light of the evil of sin, but God abhors it. This explains the terrible executions of the nations which had filled the measure of their sins.

6. Moses sent … Eleazar the priest, to the war—Although it is not expressly mentioned, it is highly probable that Joshua was the general who conducted this war. The presence of the priest, who was always with the army (De 20:2), was necessary to preside over the Levites, who accompanied the expedition, and to inflame the courage of the combatants by his sacred services and counsels.

holy instruments—As neither the ark nor the Urim and Thummim were carried to the battlefield till a later period in the history of Israel, the "holy instruments" must mean the "trumpets" (Nu 10:9). And this view is agreeable to the text, by simply changing "and" into "even," as the Hebrew particle is frequently rendered.