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 Therefore, this reason is obsolete.
(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. The second kind of war is an "optional war" (milchemet r'shut). This was a war that a king of Israel wanted to fight, with the authorization of the Sanhedrin, the Supreme Court of ancient Israel. 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.
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.
(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.'
Jewish tradition acknowledges that there is a time for war and a time for peace, but sometimes determining those times is difficult:
(א) לַכֹּ֖ל זְמָ֑ן וְעֵ֥ת לְכָל־חֵ֖פֶץ תַּ֥חַת הַשָּׁמָֽיִם׃ (ס) (ב) עֵ֥ת לָלֶ֖דֶת וְעֵ֣ת לָמ֑וּת עֵ֣ת לָטַ֔עַת וְעֵ֖ת לַעֲק֥וֹר נָטֽוּעַ׃ (ג) עֵ֤ת לַהֲרוֹג֙ וְעֵ֣ת לִרְפּ֔וֹא עֵ֥ת לִפְר֖וֹץ וְעֵ֥ת לִבְנֽוֹת׃ (ד) עֵ֤ת לִבְכּוֹת֙ וְעֵ֣ת לִשְׂח֔וֹק עֵ֥ת סְפ֖וֹד וְעֵ֥ת רְקֽוֹד׃ (ה) עֵ֚ת לְהַשְׁלִ֣יךְ אֲבָנִ֔ים וְעֵ֖ת כְּנ֣וֹס אֲבָנִ֑ים עֵ֣ת לַחֲב֔וֹק וְעֵ֖ת לִרְחֹ֥ק מֵחַבֵּֽק׃ (ו) עֵ֤ת לְבַקֵּשׁ֙ וְעֵ֣ת לְאַבֵּ֔ד עֵ֥ת לִשְׁמ֖וֹר וְעֵ֥ת לְהַשְׁלִֽיךְ׃ (ז) עֵ֤ת לִקְר֙וֹעַ֙ וְעֵ֣ת לִתְפּ֔וֹר עֵ֥ת לַחֲשׁ֖וֹת וְעֵ֥ת לְדַבֵּֽר׃ (ח) עֵ֤ת לֶֽאֱהֹב֙ וְעֵ֣ת לִשְׂנֹ֔א עֵ֥ת מִלְחָמָ֖ה וְעֵ֥ת שָׁלֽוֹם׃
(1) A season is set for everything, a time for every experience under heaven: (2) A time for being born and a time for dying, A time for planting and a time for uprooting the planted; (3) A time for slaying and a time for healing, A time for tearing down and a time for building up; (4) A time for weeping and a time for laughing, A time for wailing and a time for dancing; (5) A time for throwing stones and a time for gathering stones, A time for embracing and a time for shunning embraces; (6) A time for seeking and a time for losing, A time for keeping and a time for discarding; (7) A time for ripping and a time for sewing, A time for silence and a time for speaking; (8) A time for loving and a time for hating; A time for war and a time for peace.
(ד) וְכִתְּת֨וּ חַרְבוֹתָ֜ם לְאִתִּ֗ים וַחֲנִיתֽוֹתֵיהֶם֙ לְמַזְמֵר֔וֹת לֹא־יִשָּׂ֨א ג֤וֹי אֶל־גּוֹי֙ חֶ֔רֶב וְלֹא־יִלְמְד֥וּ ע֖וֹד מִלְחָמָֽה׃ (פ)
(4) They shall beat their swords into plowshares And their spears into pruning hooks: Nation shall not take up Sword against nation; They shall never again know war.
(ג) וְכִתְּת֨וּ חַרְבֹתֵיהֶ֜ם לְאִתִּ֗ים וַחֲנִיתֹֽתֵיהֶם֙ לְמַזְמֵר֔וֹת לֹֽא־יִשְׂא֞וּ גּ֤וֹי אֶל־גּוֹי֙ חֶ֔רֶב וְלֹא־יִלְמְד֥וּן ע֖וֹד מִלְחָמָֽה׃
(3) They shall beat their swords into plowshares And their spears into pruning hooks. Nation shall not take up Sword against nation; They shall never again know war.