9 When you go into battle in your own land against an enemy who is oppressing you, sound a blast on the trumpets. Then you will be remembered by the Lord your God and rescued from your enemies.
6 Moses sent them into battle, a thousand from each tribe, along with Phinehas son of Eleazar, the priest, who took with him articles from the sanctuary and the trumpets for signaling.
(Read Numbers 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.
Matthew Henry's Commentary on Numbers 10:9
Commentary on Numbers 10:1-10
(Read Numbers 10:1-10)
Here are directions concerning the public notices to be given the people by sound of trumpet. Their laws in every case were to be Divine, therefore, even in this matter Moses is directed. These trumpets typify the preached gospel. It sounds an alarm to sinners, calls them to repent, proclaims liberty to the captives and slaves of Satan, and collects the worshippers of God. It directs and encourages their heavenly journey; stirs them up to combat against the world and sin, encouraging them with the assurance of victory. It leads their attention to the sacrifice of Christ, and shows the Lord's presence for their protection. It is also necessary that the gospel trumpet give a distinct sound, according to the persons addressed, or the end proposed; whether to convince, humble, console, exhort, reprove, or teach. The sounding of the trumpet of the gospel is God's ordinance, and demands the attention of all to whom it is sent.