The 191st mitzvah is that we are commanded to appoint a kohen to say before the people the pre-war speech.1 Anyone who is not fit to fight in the war because of weakness2 or because they are constantly thinking of things that prevent a proper involvement in battle — i.e., the three categories3 of people mentioned in Scripture — should return home. Only then do they begin the war.
This kohen is called, "Moshuach Milchamah" ["anointed for war"]. He gives his speech in the exact words written in the Torah,4 and adds his own words to prepare the people for war and bring them to be ready to sacrifice their lives for the triumph of G‑d's religion and revenge against those who foolishly reject it5 and destroy the order of civilization.
The source of this commandment is G‑d's statement6 (exalted be He), "When you approach [the place of] battle, the kohen shall step forward ...."7
He then commands that it be announced among the soldiers that the following should return from the battle: the faint-hearted; one who has built a new house and not begun to live there; one who has planted a vineyard and not eaten [from its first crop]; and one who has betrothed a woman and not married her. This is explained in Scripture in G‑d's statement8 beginning with the words, "The officers shall then speak..." Our Sages said in the Gemara,9 "The phrase, 'The officers shall then speak,' means that the kohen speaks and the officers then announce [so that all can hear]."
All the above — i.e., the speech of the Moshuach Milchamah and the announcements before the battle — apply only to a milchemes reshus [optional war]. None of this, not the speech nor the announcement, is done for a milchemes mitzvah [obligatory war], as explained in the eighth chapter of tractate Sotah. The details of this mitzvah are explained there.
