528. No Survivors: The prohibition against leaving Canaanite combatants alive
…you shall not leave any of them alive. (Deuteronomy 20:16)
There were seven Canaanite nations that inhabited the land of Israel, whom the Jews were commanded to eradicate. However, Rambam in the Mishneh Torah (Hilchos Melachim 6:1) points out that even in these cases, we were to extend an offer of peace. We see this from the fact that Moshe offered peace to Sichon, king of Emor, before going to war (Numbers 21 and Deuteronomy 2). If the seven nations accepted Jewish rule and to observe the seven universal (Noachide) laws, they were not to be harmed in any way.
This is an extremely difficult mitzvah for us to understand in our day and age. The reason for this mitzvah is what we discussed back in Mitzvah #425: the Torah and the Books of the Prophets frequently tell us how these nations polluted the land of Israel with idolatry and immorality. We also see from Tanach how their continuing presence when they were not successfully driven out inevitably led to them entrapping the Jews in their corruption. Ultimately, we have no choice but to defer to God's Divine wisdom on such matters.
This mitzvah applies to both men and women in all times and places. It is discussed in the Talmud in tractate Sotah on page 35b. It is codified in the Mishneh Torah in the fifth chapter of Hilchos Melachim. This mitzvah is #49 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos; it is not listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.