Deuteronomy 20:9 - Ellicott's Commentary On The Whole Bible

Captains of the armies — i.e., special leaders for the campaigns, whose command would probably cease when it was over. We may suppose from 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 so far as this, that 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 number, it would remove many difficulties; for the whole thousand would very rarely be in the field together, and the contingent sent by a given “thousand” might consist of a very few men. If, therefore, the contingent of sixty “thousands” were to be described as 60,000, and the sixty companies were all cut up or annihilated, it might be reported as a slaughter of 60,000 men, while the lives actually lost would be nothing like so many.