There are three kinds of war in Jewish ethics.
1. A "required war" (milchemet chova) is a war explicitly commanded in the Torah. It applied to the seven nations of the Canaanite region and to them alone. It is not only war but genocide. This category may not be applied to anyone else. Therefore, this reason is obsolete.
(א) כִּ֤י יְבִֽיאֲךָ֙ ה' אֱלֹקֶ֔יךָ אֶל־הָאָ֕רֶץ אֲשֶׁר־אַתָּ֥ה בָא־שָׁ֖מָּה לְרִשְׁתָּ֑הּ וְנָשַׁ֣ל גּֽוֹיִם־רַבִּ֣ים ׀ מִפָּנֶ֡יךָ הַֽחִתִּי֩ וְהַגִּרְגָּשִׁ֨י וְהָאֱמֹרִ֜י וְהַכְּנַעֲנִ֣י וְהַפְּרִזִּ֗י וְהַֽחִוִּי֙ וְהַיְבוּסִ֔י שִׁבְעָ֣ה גוֹיִ֔ם רַבִּ֥ים וַעֲצוּמִ֖ים מִמֶּֽךָּ׃ (ב) וּנְתָנָ֞ם ה' אֱלֹקֶ֛יךָ לְפָנֶ֖יךָ וְהִכִּיתָ֑ם הַחֲרֵ֤ם תַּחֲרִים֙ אֹתָ֔ם לֹא־תִכְרֹ֥ת לָהֶ֛ם בְּרִ֖ית וְלֹ֥א תְחָנֵּֽם׃
(1) When the Eternal your God brings you to the land that you are about to enter and possess, and He dislodges many nations before you—the Hittites, Girgashites, Amorites, Canaanites, Perizzites, Hivites, and Jebusites, seven nations much larger than you— (2) and the Eternal your God delivers them to you and you defeat them, you must doom them to destruction: grant them no terms and give them no quarter.
2. An "optional war" (milchemet r'shut) was a war that a king of Israel wanted to fight, with the authorization of the Sanhedrin. It required 71 out of 120 rabbis to authorize it. These wars refer to the wars of King David. Neither a king of Israel nor the Sanhedrin exist today, rendering this reason also obsolete.
(ד) וּמוֹצִיא לְמִלְחֶמֶת הָרְשׁוּת עַל פִּי בֵית דִּין שֶׁל שִׁבְעִים וְאֶחָד. וּפוֹרֵץ לַעֲשׂוֹת לוֹ דֶרֶךְ, וְאֵין מְמַחִין בְּיָדוֹ. דֶּרֶךְ הַמֶּלֶךְ אֵין לוֹ שִׁעוּר. וְכָל הָעָם בּוֹזְזִין וְנוֹתְנִין לְפָנָיו, וְהוּא נוֹטֵל חֵלֶק בָּרֹאשׁ.
(4) [The King] may lead [the people] out to an optional war with the consent of the court of seventy-one. He can break through [private property] to build a road, and none may object. The king's road has no measure. And everything that the nation plunders, they place in front of him, and he takes the first portion.
3. The third kind of war is a war waged in defense, a "mitzvah war." It still applies today.
(9) When you are at war in your land against an aggressor who attacks you, you shall sound short blasts on the trumpets, that you may be remembered before the Eternal your God and be delivered from your enemies.
(ד) צָרוֹר אֶת הַמִּדְיָנִים, לָמָּה (במדבר כה, יח): כִּי צֹרְרִים הֵם לָכֶם, מִכָּן אָמְרוּ חֲכָמִים בָּא לְהָרְגֶּךָ הַשְׁכֵּם לְהָרְגוֹ.
(4) "Assail the Midianites" - Why? "For they assailed you" (Numbers 25:18). From here the sages said, 'rise and kill the one who comes to kill you.'
When you take the field against your enemies, and see horses and chariots—forces larger than yours—have no fear of them, for the LORD your God, who brought you from the land of Egypt, is with you. Before you join battle, the priest shall come forward and address the troops. He shall say to them, “Hear, O Israel! You are about to join battle with your enemy. Let not your courage falter. Do not be in fear, or in panic, or in dread of them. For it is the LORD your God who marches with you to do battle for you against your enemy, to bring you victory.” Then the officials shall address the troops, as follows: “Is there anyone who has built a new house but has not dedicated it? Let him go back to his home, lest he die in battle and another dedicate it. Is there anyone who has planted a vineyard but has never harvested it? Let him go back to his home, lest he die in battle and another harvest it. Is there anyone who has paid the bride-price for a wife, but who has not yet married her? Let him go back to his home, lest he die in battle and another marry her.” The officials shall go on addressing the troops and say, “Is there anyone afraid and disheartened? Let him go back to his home, lest the courage of his comrades flag like his.”
Ethical conduct in wartime:
(יג) כָּל הַיָּכוֹל לְהַצִּיל בְּאֵיבָר מֵאֵיבָרָיו וְלֹא טָרַח בְּכָךְ אֶלָּא הִצִּיל בְּנַפְשׁוֹ שֶׁל רוֹדֵף וַהֲרָגוֹ הֲרֵי זֶה שׁוֹפֵךְ דָּמִים וְחַיָּב מִיתָה אֲבָל אֵין בֵּית דִּין מְמִיתִין אוֹתוֹ:
If one is able to save the victim at the cost of only a limb of the pursuer and does not take the trouble to do so but saves the victim at the cost of the pursuer's life, he is deemed a shedder of blood, and he deserves to be put to death. He may not, however, be put to death by the court.
(ח) אֵין קוֹצְצִין אִילָנֵי מַאֲכָל שֶׁחוּץ לַמְּדִינָה וְאֵין מוֹנְעִין מֵהֶם אַמַּת הַמַּיִם כְּדֵי שֶׁיִּיבְשׁוּ. שֶׁנֶּאֱמַר (דברים כ, יט) "לֹא תַשְׁחִית אֶת עֵצָהּ". וְכָל הַקּוֹצֵץ לוֹקֶה. וְלֹא בְּמָצוֹר בִּלְבַד אֶלָּא בְּכָל מָקוֹם כָּל הַקּוֹצֵץ אִילַן מַאֲכָל דֶּרֶךְ הַשְׁחָתָה לוֹקֶה. אֲבָל קוֹצְצִין אוֹתוֹ אִם הָיָה מַזִּיק אִילָנוֹת אֲחֵרִים. אוֹ מִפְּנֵי שֶׁמַּזִּיק בִּשְׂדֵה אֲחֵרִים. אוֹ מִפְּנֵי שֶׁדָּמָיו יְקָרִים. לֹא אָסְרָה תּוֹרָה אֶלָּא דֶּרֶךְ הַשְׁחָתָה:
(8) Fruit-bearing trees must not be cut down outside of the city43Under siege in order to pain them. nor do we block their irrigation water causing the trees to dry up, as it says, “do not destroy her trees” (Deut. 20:19). Anyone who cuts down a tree receives lashes. This is not only at times of a siege, but anyone at anytime who chops down a fruit-bearing tree by for destructive purposes receives stripes. The tree may be cut down if it is damaging other trees or it is damaging another’s field, or because the tree is more valuable for its wood than its fruit. The Torah only forbids wanton destruction.
באו על עסקי נפשות יוצאין עליהן בכלי זיינן ומחללין עליהן את השבת ובעיר הסמוכה לספר אפילו לא באו על עסקי נפשות אלא על עסקי תבן וקש יוצאין עליהן בכלי זיינן ומחללין עליהן את השבת אמר רב יוסף בר מניומי אמר רב נחמן ובבל כעיר הסמוכה
Rav Yehuda, said that Rav said: ...if foreigners came with regard to lives, they may go out against them with their weapons, and they may desecrate Shabbat due to them. And with regard to a town that is located near the border, even if they did not come with regard to lives, but rather with regard to matters of hay and straw, i.e., to raid and spoil the town, they may go out against them with their weapons, and they may desecrate Shabbat due to them, as the border must be carefully guarded, in order to prevent enemies from gaining a foothold there. Rav Yosef bar Manyumi said that Rav Naḥman said: And Babylonia is considered like a town located near the border, and war may be waged there on Shabbat even if the foreigners came for financial gain.
(10) When you take the field against your enemies, and the Eternal your God delivers them into your power and you take some of them captive, (11) and you see among the captives a beautiful woman and you desire her and would take her to wife, (12) you shall bring her into your house, and she shall trim her hair, pare her nails, (13) and discard her captive’s garb. She shall spend a month’s time in your house lamenting her father and mother; after that you may come to her and possess her, and she shall be your wife.
What limits does the Torah put on a soldier's conduct from the preceding three passages?
(י) כָּל אֵלּוּ הַדְּבָרִים לְפִי מַה שֶּׁיִּרְאֶה הַדַּיָּן שֶׁזֶּה רָאוּי לְכָךְ וְשֶׁהַשָּׁעָה צְרִיכָה. וּבַכּל יִהְיוּ מַעֲשָׂיו לְשֵׁם שָׁמַיִם וְאַל יִהְיֶה כְּבוֹד הַבְּרִיּוֹת קַל בְּעֵינָיו שֶׁהֲרֵי הוּא דּוֹחֶה אֶת לֹא תַּעֲשֶׂה שֶׁל דִּבְרֵיהֶם וְכָל שֶׁכֵּן כְּבוֹד בְּנֵי אַבְרָהָם יִצְחָק וְיַעֲקֹב הַמַּחֲזִיקִין בְּתוֹרַת הָאֱמֶת שֶׁיִּהְיֶה זָהִיר שֶׁלֹּא יַהֲרֹס כְּבוֹדָם אֶלָּא לְהוֹסִיף בִּכְבוֹד הַמָּקוֹם בִּלְבַד שֶׁכָּל הַמְבַזֶּה אֶת הַתּוֹרָה גּוּפוֹ מְחֻלָּל עַל הַבְּרִיּוֹת וְהַמְכַבֵּד אֶת הַתּוֹרָה גּוּפוֹ מְכֻבָּד עַל הַבְּרִיּוֹת. וְאֵין כְּבוֹד הַתּוֹרָה אֶלָּא לַעֲשׂוֹת עַל פִּי חֻקֶּיהָ וּמִשְׁפָּטֶיהָ:
Whatever [the judge] does should be for the sake of Heaven, and human dignity should not be light in his eyes... He should be careful not to do anything to injure their dignity. His sole concern should be to enhance the glory of God, for whoever dishonors the Torah is himself dishonored by men, and whoever honors the Torah is himself honored by men. To honor the Torah means to follow its statutes and laws.