When you are about to go into battle, the priest is to come forward and address the army,
2 And it shall be, when ye are come nigh. God commits the duty of exhortation to the priests, when the time of the conflict shall have arrived. But we gather from the expressions used that this passage is supplementary to the First Commandment, for it contains no more than that the priest should encourage the Israelites to confidence, the ground of which is declared...
Israel was at this time to be considered rather as a camp than as a kingdom, entering upon an enemy's country, and not yet settled in a country of their own; and, besides the war they were now entering upon in order to their settlement, even after their settlement they could neither protect nor enlarge their coast without hearing the alarms of war.
Commenting on Deuteronomy 20:1-9
When all things are preparing for it, and it seems unavoidable: that the priest shall approach and speak unto the people; not any priest, but one appointed for this service; who is called the anointed of war, as Jarchi and Aben Ezra observe, and concerning whom Maimonides (d) is more particular; he says,"they appoint a priest to speak to the people at the time of...