Jamieson, Fausset and Brown on Numbers 10:9

9. If ye go to war--In the land of Canaan, either when attacked by foreign invaders or when they went to take possession according to the divine promise, "ye [that is, the priests] shall blow an alarm." This advice was accordingly acted upon (Nu 31:6; 2Ch 13:12); and in the circumstances it was an act of devout confidence in God. A solemn and religious act on the eve of a battle has often animated the hearts of those who felt they were engaged in a good and just cause; and so the blowing of the trumpet, being an ordinance of God, produced that effect on the minds of the Israelites. But more is meant by the words--namely, that God would, as it were, be aroused by the trumpet to bless with His presence and aid.