Military battles are no pleasure. The troops are likely to be apprehensive. Killing others is morally repugnant, even if they are the enemy.
Facing mortal danger is frightening, even if one is a courageous hero. Morale is a major problem, and flags, banners and bugles are no guarantee that an army will have the nerve to go ahead.
Hence ancient Israel had a special official, the kohen mashu’ach lamilchamah, “the priest anointed for battle”, whose role was to address the people and urge them not to be afraid, because God was with them (Deut. 20:3-4).
From Rashi we learn that this passage can be read on two levels – not only was God on side, but the Ark of the Covenant went with the army as visible evidence of Divine support.