Parshas Mattos:Kashering Keilim by Midyan

The Ramban asks why did B'nei Yisroel only get the mitzvah of kashering the keilim by Milchemes Midyan and not previously after Milchemes Sichon V'Og.

The Ramban answers that during war all issurim are muttar. We learn this from the possuk in Va'eschanan "U'batim mileim kol tuv". The gemara in Chullin 17a writes that even chazir is muttar during battle. Therefore, by Milchemes Sichon V'Og the b'nei Yisroel were allowed to use the pots they took. However, the Ramban explains that this heter only applies by kibush Eretz Yisroel. Milchemes Midyan was not kibush EY (ayin http://www.divreichaim.blogspot.com/ for the Rogotchover's mehalech in being magdir this war). Therefore, issurim were assur and mimeilah they needed to be told to kasher the pots.

L'ma'aseh the Ramban is l'shitaso in Parshas Va'Eschanan. There the Ramban explains that the heter b'sha'as milchama applies to everyone and even after the war the spoils of war are muttar even if they are assur (orlah, chazir). However, the Rambam disagrees. The Rambam paskens (Melachim 8:1) that this heter only applies to the army and b'sha'as hadechak when they are hungry. (In fact the Chasam Sofer in Chullin equates the heter of issurim to the heter of yifas toar-i.e. dibrah torah kneged yetzer hara). The Minchas Chinuch in Shoftim discusses this machlokes Rambam and Ramban.

According to the Rambam how do you answer the Ramban's kasha-why didn't  the get a mitzvah of kashering keilim by Sichon V'Og.

There are several ways to answer this kasha.

1) Daas Zikeinim writes that Sichon V'Og took place in the fields so there were no keilim. Midyan took place in the cities where they took keilim from the houses.

2) Several Achronim (Lev Aryeh in Chullin 17a and Tzlach in Chullin) write the Rambam is l'shitaso. In Pesachim there is a machlokes if ta'am k'ikar is min hatorah or midirabanan. Rabbi Akiva holds it is min haTorah and the rayah is that we had to kasher keilim of Midyan. The Chachamim hold klei midyan are no rayah since that whole sugyah is a chiddush cause keilim were nosein taam lifgam. The Rambam paskens ta'am kikar is d'rabanan so he holds klei midyan were a chiddush. Once you say it's a chiddush then you can't ask why davka by Midyan they got the mitzvah-the whole thing is a chiddush.
The Achronim want to take it a step farther and say that the Ramban holds ta'am kikar is d'oreisa, and mimeila he has to hold by milchama that all issurim are muttar in order to answer his kasha. However, I saw the Chasam Sofer holds the Ramban says ta'am kikar is dirabanan.

3) The Chavtzeles Hasharon doesn't like this pshat because the Pri Megadim says that even though the Rambam holds ta'am kikar is dirabanan, nevertheless kashering keilim is d'oreisa. So again why didn't they have the mitzvah by Sichon v'Og.

He answers that the Chazon Ish (OC) writes that Moshe got all the mitzvos at Har Sinai but was not told to tell B'nei Yisroel certain mitzvos until later. Could be this is one of those mitzvos and HKBH decided not to tell B'nei Yisroel until Midyan for some reason only known to HKBH.