Deuteronomy 21:10

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 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).

כִּֽי־תֵצֵ֥א לַמִּלְחָמָ֖ה עַל־אֹיְבֶ֑יךָ וּנְתָנ֞וֹ יְהֹוָ֧ה אֱלֹהֶ֛יךָ בְּיָדֶ֖ךָ וְשָׁבִ֥יתָ שִׁבְיֽוֹ׃

When thou goest forth to war against thy enemies, and the Lord thy God has delivered them into thy hands, and thou hast taken them captive,