Ramban discusses the military levy in Midian and is attentive to the deeper significance of the number and the structure of the campaign. His comments are useful for a more developed Torah essay, even where he does not foreground the exact '1,000 and 1,000' midrash as strongly as Rashi.
וַיִּשְׁלַח אֹתָם מֹשֶׁה אֶלֶף לַמַּטֶּה – לֹא שָׁלַח שָׁם כָּל עַם הַצָּבָא, וְאַף עַל פִּי שֶׁהָיוּ הַמִּדְיָנִים עַם רַב וְהֶעָרִים בְּצוּרוֹת גְּדוֹלוֹת מְאֹד. וְהַטַּעַם, כִּי הַנִּכְשָׁלִים בִּבְנוֹת מוֹאָב הָיוּ רַבִּים וְאֵינָם רְאוּיִים לְנִקְמַת ה', עַל כֵּן בָּחֲרוּ אֲנָשִׁים צַדִּיקִים וִידוּעִים לְשִׁבְטֵיהֶם. וְהִנֵּה מֹשֶׁה לֹא צִוָּה אוֹתָם מַה יַעֲשׂוּ, רַק אָמַר לָהֶם לָתֵת נִקְמַת ה' בְּמִדְיָן (במדבר ל״א:ג'), וְחָשַׁב שֶׁלֹּא יַשְׁאִירוּ בָּהֶם שָׂרִיד וּפָלִיט כְּנִקְמַת עֲמָלֵק אוֹ כְּנִקְמַת שִׁבְעָה עֲמָמִים. וְכַאֲשֶׁר רָאָה שֶׁהִשְׁאִירוּ הַנָּשִׁים וְהַטַּף וְהַבְּהֵמָה, קָצַף עַל הַנָּשִׁים הַיּוֹדְעוֹת מִשְׁכַּב זָכָר, כִּי רָאוּי לִפְקוּדֵי הַחַיִל לְהָרְגָן תְּחִלַּת כָּל דָּבָר, גַּם לִנְקָמָה גַּם לְדִין הַתּוֹרָה ״וְאֶת הַבְּהֵמָה תַּהֲרֹגוּ״ (ויקרא כ' ט״ו). וְכֵיוָן שֶׁרָאָה שֶׁהָעָם חָפֵץ לִשְׁלֹל, מָחַל עַל הַטַּף בַּנָּשִׁים וְעַל הַשָּׁלָל. וְיִתָּכֵן עוֹד שֶׁנֹּאמַר כִּי מֹשֶׁה נִצְטַוָּה ״צָרוֹר אֶת הַמִּדְיָנִים״ (במדבר כ״ה:י״ז) וּ"נְקֹם נִקְמַת בְּנֵי יִשְׂרָאֵל״ (לפנינו פסוק ב'), וְשָׁלַח שָׁם מוּעָטִים לְהַכּוֹת בְּעָרֵי הַפְּרָזִי וְכָל עֵץ טוֹב לְהַפִּיל וְכָל מַעְיַן מַיִם לִסְתֹּם וְכָל חֶלְקָה טוֹבָה לְהַכְאִיב בָּאֲבָנִים כְּמִנְהַג הַשּׁוֹלְלִים, וְלֹא צִוָּה לָהֶם דָּבָר רַק לְהִנָּקֵם כַּאֲשֶׁר תִּמְצָא יָדָם. וְהַשֵּׁם יִתְבָּרַךְ אֲשֶׁר לוֹ הַמִּלְחָמוֹת נָתַן מִדְיָן וּמַלְכֵיהֶם וְעָרֵיהֶם בְּיָדָם. וְעַל כֵּן קָצַף עַל הַנָּשִׁים הַיּוֹדְעוֹת מִשְׁכַּב זָכָר, לֹא דָּבָר אַחֵר, וְצִוָּה בַּטַּף הַזְּכָרִים לִנְקָמָה. וְהִנֵּה קָצַף עַל פְּקוּדֵי הַחַיִל וְחָלַק כָּבוֹד לְפִינְחָס, כִּי הַשֵּׁם נָתַן לוֹ אֶת בְּרִיתוֹ שָׁלוֹם. וְשָׁנִינוּ בְּסִפְרֵי (מטות מ״ג): אָמַר לוֹ פִּינְחָס, כְּשֵׁם שֶׁפְּקַדְתָּנוּ כֵּן עָשִׂינוּ. וְלֹא יָדַעְתִּי מַהוּ, שֶׁלֹּא פָּקַד אוֹתָם דָּבָר בַּכָּתוּב, וְאִלּוּ פָּקַד אוֹתָם וְעָשׂוּ, הֵיאַךְ יִכְעוֹס? וְאִם פָּקַד אוֹתָם, חָלִילָה שֶׁיַּעֲבֹר פִּינְחָס מִצְוָתוֹ, כִּי שָׁאוּל אִבֵּד מַלְכוּתוֹ עַל זֶה (שמואל א' ט״ו י״א). אֲבָל הָעִנְיָן כַּאֲשֶׁר אָמַרְתִּי, שֶׁפְּקָדָם לָתֵת נִקְמַת ה' בְּמִדְיָן, וְאָמַר פִּינְחָס: נְקָמָה גְדוֹלָה עָשִׂינוּ בָהֶם. וְאֶפְשָׁר לְפָרֵשׁ שֶׁאָמַר לוֹ: כְּשֵׁם שֶׁפְּקַדְתָּנוּ מֵהַר סִינַי בְּדִין הַתּוֹרָה כִּי (תצור) תִּקְרַב אֶל עִיר" וְגוֹ' (דברים כ' י'-י״ח), כָּךְ עָשִׂינוּ בָהֶם. וּמֹשֶׁה קָצַף עַל הַנָּשִׁים הַגְּדוֹלוֹת בַּעֲבוּר ״הֵן הֵנָּה הָיוּ לִבְנֵי יִשְׂרָאֵל״ (במדבר ל״א:ט״ז), וְהוֹסִיף כָּל זָכָר בַּטַּף בַּעֲבוּר הַנְּקָמָה:
AND MOSES SENT THEM, A THOUSAND OF EVERY TRIBE. He did not send all the members of the army there, although the Midianites were a large people, and their cities were fortified, and very large. The reason for this is that those who had sinned with the Moabite women were many, and they were not fit to execute the vengeance of the Eternal; therefore they chose those men who were known amongst their tribes as righteous men. Now Moses did not command them what to do, but merely told them to execute the vengeance of the Eternal on Midian, and he thought that they would let none of them remain or escape, but [would execute the vengeance of the Eternal] like the vengeance [they were commanded] in connection with Amalek or the vengeance [they were commanded] in connection with the seven nations [of the land of Canaan]. And therefore when Moses saw that they had spared the women and children and the cattle, he was angry about [the fact that they had left alive] the women who had had sexual intercourse, because the officers of the army should have killed them as the very first thing, both as execution of the vengeance [which they were now commanded to do], and in performance of the general law of the Torah [that a living thing which causes a man to sin must be killed, as it is said], and ye shall slay the beast. And when he saw that the people wanted [to keep] the spoil, he remitted [and allowed them to keep] the female children and the spoil. It is also possible to explain that Moses was [only] commanded to Harass the Midianites, and Avenge the children of Israel of the Midianites, [but was not commanded to destroy them all like Amalek and the seven nations]. Therefore he sent only a small contingent [twelve thousand men] to strike at the unwalled towns and fell every good tree, and stop all fountains of water, and mar every good piece of land with stones as spoilers usually do, and he did not give them any specific commands except that they should execute whatever vengeance they were able to do — and G-d, blessed be He, Who controls [the course of all] battles, gave the Midianites and their [five] kings and cities into their hands. Therefore Moses was only angry about [their sparing] the women who had had sexual intercourse, and [he was] not angry about anything else, and his command [to kill] the male children was an [additional measure of] retributive punishment. Now Moses was wroth with the officers of the host but showed honor to Phinehas [by not criticizing him, too], because G-d had given unto him His covenant of peace. And in the Sifre we are taught: “Phinehas said to Moses: As you have commanded us, so have we done.” But I do not know what this statement means, since according to the Scriptural verses [here] Moses did not command them anything specific, for had he commanded them something which [as Phinehas said] they fulfilled, how could he [Moses] have been angry with them? [We cannot say that he was angry because they did not do everything that he told them to,] because had he commanded them, Heaven forbid that [we say that] Phinehas would transgress his command! Saul lost his kingdom on account of such disobedience! But the [explanation of the matter] is as I have said, that Moses [merely] commanded them to execute the vengeance of the Eternal on Midian, and Phinehas told him: “We have executed great vengeance on them.” It is also possible to explain that Phinehas told Moses: “Just as you charged us with the law of the Torah given at Mount Sinai, namely, When thou shalt besiege a city etc. so have we done to them.” And Moses was angry about [their sparing] the adult women, because, Behold, these caused the children of Israel etc., and he added [the command to kill] every male among the little ones in order to complete the retribution.