Numbers 31:6

And Moses sent them to the war, a thousand of every tribe, them and Phinehas the son of Eleazar the priest, to the war, with the holy instruments, and the trumpets to blow in his hand. Moses sent them ... and Phineas the son of Eleazar the priest. 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 (Deuteronomy 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. Since neither the ark nor the Urim and Thummim were carried to the battlefield until a later period in the history of Israel, the "holy instruments" must mean the "trumpets" (Numbers 10:9). And this view is agreeable to the text by simply changing "and" into 'even,' as the Hebrew particle is frequently rendered.