correction concerning milchamah

I realized that I was very wrong about yesterday's post.  I assumed that the war with midian had a din of milchemes mitzvah.  After all, the war was commanded by Hashem.  Apparently that assumption was totally incorrect.

The Ramban, in his hasagos to the sefer hamitzvos, says the rambam forgot to count a mitzvah in this weeks parshah - the mitzvah to besiege a city from only 3 sides (leaving the fourth side open for people to flee and avoid death) in a milchemes reshus.  This mitzvah is learned from milchemes midian, where chazal darshen the pasuk, "bnei yisrael attacked midian in the way hashem commanded" to be referring to this halacha of leaving the fourth side open.  The ramban clearly sees milchemes midian as a paradigm for milchemes reshus lidoros.

(The rambam doesnt disagree with the halacha - he quotes it in hilchos melachim.  he just doesnt think it merits to be counted as its own mitzvah.  As the megillas esther points out, the rambam probably felt it was just a detail in halacha subsumed under the broader mitzvah of milchemes reshus. Further, given that the din is learned out from a derasha, I think it would be a violation of the rambam's shoroshim to count it as its own mitzvah - yesh ladun more on this issue.)

Furthermore, it is prohibited to loot or leave the women and children alive in a milchemes mitzvah - that bnei yisrael did so here would prove that milchemes midian was a milchemes reshus.

So before I discuss this point more, getting back to the rambam yesterday, I dont know how he would deal with the sifri that says here that the leviim fought EVEN in a milchemes reshus - kol shekain they would fight in a milchemes mitzvah.  I note that internally, the Rambam is unclear.  While he does say in the last halacha in hilchos shemittah viyovel that leviim arent "orchin milchamah", it is very strange that this halacha is mentioned nowhere (as far as I know) in hilchos melachim, where the rambam discusses the laws of war in detail.  It is very troubling that its not there - that is its primary place, and this requires explanation.  vihadavar tzarich talmud.

(as an aside, see the last hasagas haraavad in hilchos shemittah - there is a machlokes between the rambam and the raavad whether leviim are allowed to partake in loot from a conquest outside of israel (it is definitely assur in EY bc. the pasuk says leviim dont take nachala or loot - hashem is their nachala).  according to the raavad, the leviim who fought midian should be excluded from the count of looters - but they aren't!!)

So this is very strange - a milchemes reshus, one would think from the name, is a voluntary war.  How can a war commanded by G-d be voluntary?

I'm too lazy / have not done enough research to justify writing what I think the answer to this question is.  I'll leave it for now as food for thought (and I'm always open to suggestions), and I'll just say that I think there is (potentially) a lot of mussar in the answer.