כי תצא למלחמה. בְּמִלְחֶמֶת הָרְשׁוּת הַכָּתוּב מְדַבֵּר, שֶׁבְּמִלְחֶמֶת אֶרֶץ יִשְׂרָאֵל אֵין לוֹמַר וְשָׁבִיתָ שִׁבְיוֹ, שֶׁהֲרֵי כְבָר נֶאֱמַר (דברים כ') לֹא תְחַיֶּה כָּל נְשָׁמָה: כי תצא למלחמה WHEN THOU GOEST FORTH TO WAR — Scripture is speaking here of a war that is not obligatory upon the Israelites (Sifrei Devarim 211:1), for in regard to a war that was waged against the inhabitants of Erez Israel, Scripture could not possibly say, “and thou hast captured captives”, since it has already been stated regarding them, (Deuteronomy 20:16) “[But of the cities of those people ...] you shall not allow any soul to live”.
ושבית שביו. לְרַבּוֹת כְּנַעֲנִים שֶׁבְּתוֹכָהּ וְאַעַ"פִּ שֶׁהֵם מִשִׁבְעָה אֻמּוֹת (ספרי; סוטה ל"ה): ושבית שביו AND THOU HAST CAPTURED CAPTIVES — These apparently redundant words are intended to include Canaanite people living in it (in a city outside Canaan), that it is allowed to capture them although they belong to the seven nations (Sifrei Devarim 211:4; Sotah 35b).