1. I have to thank Havolim for mentioning a beautiful sevara of the Rogatchover that I otherwise might have missed. The Rogatchover notes that the Torah characterizes the fight against Midyan as an act of nekama, not a war. Placing this battle in its own halachic category explains a number of its unique characteristics:
a) On Shabbos I saw the Taz quotes a Tzeror haMor who holds there was no heter to take a yefat to'ar during this war. Tos' (Shabbos 65, also quoted in the Da'as Zekeinim al haTorah) asks why the generals were concerned lest the soldiers had sinned with hirhurei avirah -- if actually taking a yefat to'ar in battle was permitted, how could hirhur of the same be prohibited? The Tzeror haMor's approach renders this question moot. Tosfos is forced to answer that the concern was for the women who were exceptions to the rule of yefat to'ar and could not be taken.
The Taz assumes that the Tzeror haMor held there was no heter of yefat to'ar for wars waged outside Eretz Yisrael and he marshals proof otherwise. However, based on the Rogatchover one could suggest a different justification for the Tzeror haMor’s view: the battle against Midyan was not a war, but rather was an act of nekama.
(Parenthetically, from a mussar perspective it makes perfect sense that there be no heter of yefat to'ar here. The troubles with Midyan started because of the enticement of znus -- would it make sense to allow the same situation to develop? On the other hand, chassidishe seforim argue just the opposite -- the reason the soldiers at first let the women live was to prove their ability to overcome the yetzer ha'ra that had previously possessed them and do teshuvah gemurah.)
b) Ramban asks why the Torah teaches the halachos of kashering and toiveling only after the war with Midyan and not earlier, after the wars against Sichon v'Og. He answers by quoting the gemara in Chulin (17) that katli d'chaziri was permitted during wars of kibush v'chiluk -- chazir was allowed to be eaten during the wars of conquest of Eretz Yisrael. The wars against Sichon v'Og were wars of conquest, for the purpose of obtaining territory; the war against Midyan was not. Based on the Rogatchover, one can draw an even sharper distinction -- the war against Midyan wasn't a war at all.
Current events sheds some light on why some people are averse to the use (or overuse) of lomdus like this. Does calling something a "kinetic military action" (Pres. Obama's description of what we are doing in Libya) make it any less of a war than it otherwise would be? Does calling a battle an act of nekama instead of milchama change reality? Does a rose by any other name.... I love lomdus like this, but I can see why it would drive some people crazy.
2) M'inyan l'inyan -- Why did Reuvain and Gad wait until after the battle with Midyan to request that they be given the
3) Last week we touched on the question of why the generals did not step forward with their gifts to the Mishkan for the sake of a kapparah on hirhur immedately after the war ended -- why did they wait? Some of the meforshim explain that it was hearing the parsha of kasheing kelim which made them recognize the need for kapparah. A kli can be completely clean from ma'achalos asurus, yet it still needs to be kashered. It's not just mamashus of issur which is a problem, but it is also that which is absorbed, that which lies unseen below the surface, which is halachically dangerous. The generals realized that even if the army was innocent of any mamashus of aveirah, the thoughts that lurked below the surface might have a ta'am issur that needed kashering.