Is Torah Study a Part of the War Effort?

In the polemics about whether charedim should serve in the IDF, there is one source frequently quoted to “prove” that learning Torah is equally or even more important to national defense:

אָמַר רַבִּי יְהוֹשֻׁעַ בֶּן לֵוִי: מַאי דִּכְתִיב: ״עֹמְדוֹת הָיוּ רַגְלֵינוּ בִּשְׁעָרַיִךְ יְרוּשָׁלִָם״? מִי גָּרַם לְרַגְלֵינוּ שֶׁיַּעַמְדוּ בַּמִּלְחָמָה – שַׁעֲרֵי יְרוּשָׁלַםִ, שֶׁהָיוּ עוֹסְקִים בַּתּוֹרָה.

Rabbi Yehoshua ben Levi says: What is the meaning of that which is written: “Our feet were standi...