Sforno on Numbers 31:6/ספורנו על במדבר ל״א:ו׳

1וישלח אותם משה אלף אלף למטה לצבא, he sent one thousand men of each tribe.

2אותם ואת פינחס, after the 12.000 men had all been assembled he sent Pinchas to accompany them.

3לצבא וכלי הקודש וחצוצרות התרועה בידו, he sent him to the army, as head, i.e. as the priest especially appointed in every war involving invasion of foreign territory. The various holy vessels, including the trumpets would be at his disposal during that period.