Rashi on Deuteronomy 20:10/רש"י על דברים כ׳:י׳

1כי תקרב אל עיר WHEN THOU APPROACHEST UNTO A CITY [TO FIGHT AGAINST IT] — Scripture is speaking of a war which is not obligatory upon them (as was the war against the seven nations of Canaan, referred to in v. 16), as it is distinctly stated in this section (v. 15) “Thus thou shalt do unto all the cities which are very far [from thee]” etc. (Sifrei Devarim 199:1).