Tafsiran Ulangan 20:2: Rashi, Chizkuni, Ibn Ezra & Rabbeinu Bahya

כקרבכם אל המלחמה [IT SHALL BE] WHEN YE COME NIGH UNTO THE BATTLE, [THAT THE PRIEST … SHALL SPEAK] — i.e. when ye are on the point of leaving the ספר — the boundary of your land (Sifrei Devarim 191).

והיה כקרבכם אל המלחמה, “and it will be when you will be close to the battle, etc.” once the soldiers have approached the borders of the enemy, before actually engaging him in battle, Moses informs them of all the conditions which must be observed during active warfare. They are, of course, reserved for the men actually engaged in the fighting. This is clear from the wording in verse three, where only those men are being addressed. [Seeing that the Torah speaks not of a defensive war, but of a war designed to enlarge the borders of Israel, with the approval of G-d of course. Ed.]

the kohen will come forward i.e., the one specifically attached to the military [Soṭa 42a].

ונגש הכהן AND “THE” PRIEST SHALL STEP NIGH [AND SPEAK] “the” priest means, he who has been anointed for that purpose; it is the one who is termed in the Talmud משוח מלחמה, “the priest anointed for war” (Sotah 42a).