Jamieson, Fausset and Brown on Deuteronomy 20:2-4

2-4. when ye are come nigh unto the battle, that the priest shall approach and speak unto the people--Jewish writers say that there was a war priest appointed by a special ceremonial to attend the army. It was natural that the solemn objects and motives of religion should have been applied to animate patriotism, and so give additional impulse to valor; other people have done this. But in the case of Israel, the regular attendance of a priest on the battlefield was in accordance with their theocratic government, in which everything was done directly by God through His delegated ministers. It was the province of this priest to sound the trumpets (Nu 10:9; 31:6), and he had others under him who repeated at the head of each battalion the exhortations which he addressed to the warriors in general. The speech (De 20:3, 4) is marked by a brevity and expressiveness admirably suited to the occasion, namely, when the men were drawn up in line.