Tafsiran Ulangan 20:16: Chizkuni, Ibn Ezra, Rashbam & Rav Hirsch

לא תחיה כל נשמה, “you must not allow to remain for anyone to be alive;” if you were to ignore this commandment, the survivors would stir up trouble internationally, even, over having been deprived of their ancestral land. The validity of this commandment is limited to the generation Moses is addressing, i.e. the period during which the Israelites will be engaged in fighting the Canaanites in order to settle in the land promised by G-d to their patriarchs, Avraham, Yitzchok, and Yaakov. If members of these tribes had emigrated voluntarily, and at some future date return individually, and even wish to convert to Judaism this is acceptable. According to this interpretation, the Israelites under Joshua did not sin when they accepted the conversion of the Givonim, even though these had misrepresented themselves.

you must not spare It was forbidden to feed them, or give them water. The basic principle is that one was required in those days to kill them, if possible. Scripture does not here mention the Girgashites, because they were they fewest and the smallest of them.

לא תחיה כל נשמה. When you confront them in battle do not offer them peace and expulsion as in the case of other hostile nations that are not Canaanites, a procedure I (Moses) have already explained in verse 10. However, if they make the first move in approaching you, and express their willingness to become your slaves before the battle is joined you are allowed to let them live.