Albert Barnes Commentary Deuteronomy 20:2

"And it shall be, when ye draw nigh unto the battle, that the priest shall approach and speak unto the people," — Deuteronomy 20:2 (ASV)

The priest - Not the high priest, but one appointed for the purpose, and called, according to the rabbis, “the anointed of the war”: therefore, perhaps the expression of Jeremiah 6:4 and other passages, prepare you (literally consecrate) war. Thus, Phinehas went with the warriors to fight against Midian (Numbers 31:6; compare to 1 Samuel 4:4, 1 Samuel 4:11; 2 Chronicles 13:12).