John Gill Commentary Deuteronomy 20:13

"and when Jehovah thy God delivereth it into thy hand, thou shalt smite every male thereof with the edge of the sword:" — Deuteronomy 20:13 (ASV)

And when the Lord your God has delivered it into your hands ,
&c.] 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 or pestilence among the besieged, and inclining their hearts to deliver up their city:

you shall smite every male thereof with the edge of the sword ;
the men in it, grown persons, as distinguished from little ones in the next verse; because it was owing to these it was not surrendered at once, when terms of peace were offered.