Halakhah su Deuteronomio 20:17

Indeed, even if such an obligation were to exist in our day, that obligation would be severely limited in nature. Minḥat Hinnukh, no. 425, raises an obvious question. All commandments, with the exception of the prohibitions against homicide, idolatry and certain sexual offenses, are suspended for the purpose of saving a life. Actions which otherwise would be prohibited are permissible, and indeed mandatory, in the event that there exists even a remote chance that a life may be saved as a result of their performance. Obligations which are otherwise mandated are suspended in face of even possible danger to life. Failure to wage an obligatory war is not enumerated as one of the cardinal sins demanding martyrdom rather than trangression. How, then, can the Torah command us to wage war? Yet war for the conquest of Erez Yisra'el as well as for the eradication of Amalek is a mandatory duty. Warfare obviously presents the possibility of casualties and, even in the most favorable of circumstances, poses a threat to life. The scriptural phrase "va-ḥai ba-hem—and he shall live by them" (Leviticus 18:5) is understood by the Sages as suspending the yoke of the commandments when fulfillment might mean that the person so obligated might "die by them" rather than "live by them." Minḥat Hinnukh resolves the problem by explaining that the commandments concerning war are unique. Warfare, by virtue of its nature, demands that a participant's life be placed in danger. Hence, in this case, the nature of the mizvah requires that one place one's life in danger. Since that is the very essence of the obligation, the mizvah cannot be suspended in face of possible danger.⁷Cf., R. Naphtali Zevi Yehudah Berlin, Meromei Sadeh, Eruvin 45a and Kiddushin 43a; and R. Yitzchak Ze’ev Soloveitchik, Ḥiddushei Maran Riz ha-Levi al ha-Torah (Jerusalem, 5723), Parshat Beshallaḥ, p. 32. These scholars quite appropriately note that, even in the absence of a miẓvah, considerations of endangerment of self or of others are set aside in time of war simply by virtue of the “laws of war,” i.e., the Torah’s very recognition and sanction of warfare constitutes dispensation for endangerment of lives in the conduct of war.
R. Joshua Aaronberg, Dvar Yehoshu’a, II, no. 48, extends this position in stating that, since considerations of self-endangerment are set aside in the conduct of war, war may not be eschewed if avoidance of war would result in infraction of even a rabbinic prohibition. Thus, for example, the prohibition, lo titen lahem ḥaniyah be-karka may not be violated because of considerations of pikuaḥ nefesh since there exists the option of waging war in order to prevent non-Jewish occupation of land. This writer finds Rabbi Aaronberg’s thesis unconvincing. The Torah permits self-endangerment in a milḥemet miẓvah; nowhere is there the slightest hint that an otherwise non-obligatory war becomes obligatory when necessary to avoid suspension of any prohibition in the face of danger. On the contrary, the Gemara, Gittin 56a, indicates that a blemished animal might be accepted as a sacrificial offering because refusal would offend the authorities and result in danger to Jews. There is no suggestion that war, even if potentially successful, must be undertaken in order to avoid such transgression. Moreover, the Brisker Rav, in his above cited comment, explicitly writes that the commandment haḥarem taḥarimem (Deuteronomy 20:17), qua miẓvah, is suspended even in time of war.

Rav Schachter summarizes the many opinions regarding the Rambam’s celebrated omission of the mitzvah of living in Israel from his list of the 613 mitzvot. He presents opinions (cited by Sdei Chemed Ma’arechet Eretz Yisrael 2) that the Rambam believes that the mitzvah to live in Israel is rabbinic and therefore does not list it as one of the 613 mitzvot. Rav Schachter then cites the Avnei Neizer (in his aforementioned responsum), who asserts that once the Rambam counted the mitzvah of conquering the seven nations who lived in Israel prior to the conquest of Yehoshua (Devarim 20:17), he did not find it necessary to count the actual conquest and settlement as a separate mitzvah.

And that is that He commanded us to kill the seven nations that dwelled in the Land of Canaan and to destroy them, since they are the root of idolatry and its first base. And that is His, may He be exalted, saying, "you must surely annihilate them" (Deuteronomy 20:17). And He explained to us in many verses that the reason for this is so that we do not learn from their heresy. And many verses come to hint about this - meaning their killing - to strengthen it; and the war against them is a commanded war. And perhaps one might think that this commandment is not practiced for [all] the generations, since the seven nations have already ceased to exist. However it is [only] one who does not understand the topic of a practice that is practiced for the generations or that is not practiced for the generations who will think this. For it is not said about a command that is finished by the arrival of its purpose - without it being dependent upon a specific time - that it is not practiced for the generations. Rather it is practiced in each and every generation that the thing is found to be possible. Would you think that when God, may He be exalted, destroys the seed of Amalek and cuts it off with finality - as it will soon be, as He promised us by His saying, "I will surely blot out the memory of Amalek" - it is not for the generations? This would never be said! Rather it is practiced in each and every generation: Any time that the seed of Amalek is found, it is commandment to cut it off. And so too is killing the seven nations and destroying them a statement that was commanded; and it is a commanded war. And we are commanded to search for them in each and every generation until they are finished and not a single man of them remains. And so did we do in the days of David. [Then their] remnants scattered and mixed in with the [other] nations, until no root of them remained. But it is not because they have been cut off that this commandment that we have been commanded [should be considered] a commandment that is not practiced for the generations - and even after they have been finished and they have been destroyed. For these commandments are not connected to a specific time or place, like the commandments that were specific to the desert. Rather they are connected to to all times that it is found that this command is possible. And more generally, it is surely appropriate for you to understand and know the difference between commandments and that thing about which we were commanded. For, many times, the commandment will be practiced for the generations, but the thing about which we were commanded has already disappeared from certain times and places. But it does not turn into a commandment that is not practiced for the generations with the disappearance of the thing about which we were commanded. Rather it is [considered] not practiced for the generations when the matter is the opposite. And that is when certain content is in existence, but one is [only] obligated to do an action or a law at a certain time. But today it is not practiced, even though the thing is found with that matter. For example, a Levite who was disqualified in the desert is suitable today with us, as is explained in its place (Chullin 24a). And understand this principle and place it upon your heart. (See Parashat Shoftim; Mishneh Torah, Kings and Wars 5.)