Albert Barnes 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)

For examples of the employment of trumpets in war, compare marginal references and (2 Chronicles 20:28). By their employment was signified the dependence of God’s people on His aid.