There is a special mercy in appointing Phineas to this service. He it was whose zeal for GOD'S honour broke forth in a day of affliction to the deliverance of Israel. See Numbers 25:7-13.
Related Commentaries of Numbers 31:4
Numbers 31:4-6
4 Of every tribe a thousand, throughout all the tribes of Israel, shall ye send to the war.
5 So there were delivered out of the thousands of Israel, a thousand of every tribe, twelve thousand armed for war.
6 And Moses sent them to the war, a thousand of every tribe, them and Phinehas the son of Eleazar the priest, to the war, with the holy instruments, and the trumpets to blow in his hand.