Charles Ellicott Commentary Numbers 10:9

"And when ye go to war in your land against the adversary that oppresseth you, then ye shall sound an alarm with the trumpets; and ye shall be remembered before Jehovah your God, and ye shall be saved from your enemies." — Numbers 10:9 (ASV)

And if ye go to war. — Better, And when ye shall go to war. In Numbers 31:6 we read that in the war against the Midianites, Phinehas, the son of Eleazar, had these trumpets of alarm (hazozeroth) in his hand. So also Abijah, in his address to Jeroboam, before the battle, lays great stress upon the fact that Judah had on their side the priests with the trumpets of alarm (2 Chronicles 13:12; 2 Chronicles 13:14). On the other hand, the seven priests who compassed the city of Jericho carried the shophar, or keren — that is, rams’ horn — not the hazozerah, or silver trumpet.