The truth is that we can still actually translate כִּי־תֵצֵא לַמִּלְחָמָה to mean “If you go out to war upon your enemies” as Rashi states that this Possuk refers to an optional battle and it can still align with the Alter Rebbe’s translation that it refers to the battle of the Avodah of prayer.
Because although the Avodah of prayer is an obligatory battle for every single Jew, however, it is only the beginning of one’s Avodah. Soon afterwards there are two options of Avodah: to immerse into the Avodah of Torah study or the Avodah of prayer. To explain: it has already been explained in several places that there are two paths to dealing with and refining one’s Yetzer HoRa - one’s evil inclination. Either through battle or through peace.
These two paths generally mirror the two paths in Avodah: The Avodah of prayer which (as explained earlier) is an Avodah of battle, and the Avodah of Torah study which as the Possuk states: דְרָכֶיהָ דַרְכֵי־נֹעַם וְכָל־נְתִּיבוֹתֶיהָ שָלוֹם “Its ways are ways of pleasantness, and all its paths are peace”.
