Albert Barnes Commentary Numbers 31:6

"And Moses sent them, a thousand of every tribe, to the war, them and Phinehas the son of Eleazar the priest, to the war, with the vessels of the sanctuary and the trumpets for the alarm in his hand." — Numbers 31:6 (ASV)

Phinehas – He was marked out as the fitting director of the expedition by his conduct in the matter of Zimri and Cozbi.

With the holy instruments, and the trumpets – Or rather, “with the holy instruments, namely, the trumpets,” for the trumpets themselves seem to be the instruments intended.