Charles Ellicott Commentary Deuteronomy 20:9

"And it shall be, when the officers have made an end of speaking unto the people, that they shall appoint captains of hosts at the head of the people." — Deuteronomy 20:9 (ASV)

Captains of the armies—that is, special leaders for the campaigns, whose command would probably end when the campaign was over. We may suppose from the mention of the “thousands” in the army—the captain of their thousand (1 Samuel 17:18)—that the military divisions corresponded with the civil organization of the people in this respect: the men of the same “thousand,” according to Jethro’s arrangement, would be brigaded together and have one captain.

If, as is also possible, the word “thousand” in military language signifies the contingent furnished by a “thousand” in Israel, irrespective of its actual number, it would resolve many difficulties; for the entire “thousand” (as a unit) would very rarely be in the field simultaneously, and the contingent sent by any given “thousand” might consist of very few men. If, therefore, the contingent from sixty “thousands” were described as 60,000, and these sixty companies were all decimated or annihilated, it might be reported as a slaughter of 60,000 men, while the number of lives actually lost would be far fewer.