Adam Clarke on Numbers 10:9

If ye go to war - These trumpets shall be sounded for the purpose of collecting the people together, to deliberate about the war, and to implore the protection of God against their enemies.

Ye shall be remembered before the Lord - When ye decamp, encamp, make war, and hold religious festivals, according to his appointment, which appointment shall be signified to you by the priests, who at the command of God, for such purposes, shall blow the trumpets, then ye may expect both the presence and blessing of Jehovah in all that ye undertake.