When the LORD your God has delivered it into your hand, you must put every male to the sword.
They are here directed what method to take in dealing with the cities (these only are mentioned, Deu 20:10, but doubtless the armies in the field, and the nations they had occasion to deal with, are likewise intended) upon which they made war.
Commenting on Deuteronomy 20:10-20
And when the Lord thy God hath delivered it into thine hands,.... When, what with pressures without, and calamities within, the city is obliged to surrender: this is not to be imputed to the methods and arts of war used in besieging, or to the courage and skill of the besiegers; but to the power and providence of God succeeding means used, and sending famine...
Deu 20:12-14 If the hostile town, however, did not make peace, but prepared for war, the Israelites were to besiege it; and if Jehovah gave it into their hands, they were to slay all the men in it without reserve (“with the edge of the sword,” see at Gen 34:26); but the women and children and all that was in the city, all its spoil...
Commenting on Deuteronomy 20:12-14