Thank you for your reply. The subject is much clearer now. I did have 2 more questions.
1. Are there any nafka minas between a milchamas mitzvah (eg kivuch erretz yisroel) and a milchama for a mitzvah (to save Jewish lives)? By both wars a chassan will go to war.
2. Also I was wondering if a Ashes Yafes Toar is by a milchamas mitzvah, reshoos or both
Ari,
You're very welcome! Thank you for your follow up questions.
1) There are a few Nafka Mina's between Milchemos Mitzvah and Milchemos Reshus. Most Rishonim say that it seems from several sources in Chazal that all the Parshiyos in the Torah that start with "Ki Setzei" mean that it is not a required war but a Reshus. So, the Isur of cutting down fruit trees while besieging the enemy, and calling for peace before the war, and the Parshah of the Kohen who calls on those who married a wife or built a house to return to his home, are not relevant in a Mitzvah war. Presumably, in a war that has a Mitzvah but is essentially a Milchemes Reshus, if it is possible one must call the enemies for peace, and if they have reached a state of siege, one must not cut down fruit trees.
2) There is a Machlokes whether Yefas To'ar is only in the case of a Milchemes Reshus, or also in the case of a Milchemes Mitzvah. The Sifri on Shoftim and Ki Setzei (211, see also Sotah 35b) says that the Parshiyos of Keri'as Shalom (calling for peace) and Yefas To'ar are only in a Milchemes Reshus, since in a Milchemes Mitzvah in Eretz Yisrael against the seven nations, we have a Mitzvah of "Lo Sechayeh Kol Neshamah." On the other hand, the Rambam says that Yefas To'ar is even in the case of a Milchemes Mitzvah.
Best Regards,
Aharon Steiner
1) Rashi cites this from the Sifri in the beginning of Parshas Ki Setzei, and says that this must be the Pshat, since in the case of a Milchemes Mitzvah there are no captives and therefore no Eshes Yefas To'ar. The Sifri says the same by "Ki Sikrav El Ir," regarding cutting down fruit trees. The Parshah of the Kohen who sends home a first-year just-married person is obviously referring to a Milchemes Reshus, since we know that in the case of a Milchemes Mitzvah, not only do we not send home a Chasan, but we even recruit him! However, the Rambam says that even though the Kohen does not send anybody home, he still speaks in front of the troops and gives them moral support.
The Mefarshim on Rashi add that besides the captivity issue, the words "Ki Setzei" mean "if you go fight a war." The "if" statement means also Reshus.
As for the Rambam, it actually is not written explicitly in the Rambam, but it is derived from his words in the Mirkeves ha'Mishneh (Hilchos Melachim, beginning of chapter 6) and in the Minchas Chinuch (532:1).
2) Great question! I'll start with Rashi. The Mizrachi on Rashi says that if the enemy is not from the seven nations, a Yefas To'ar is permitted. The Gur Aryeh argues and says that in no case is there a Yefas To'ar in a war on Eretz Yisrael. We can assume that the same dispute can be applied to the Rambam, too.
By the way, I found some more Nafka Minas between a Milchemes Mitzvah and Reshus: (a) whether the king needs to fight in the front lines (Rambam, Hilchos Malachim 5:1); (b) whether the king needs to ask the Kohen Gadol and/or the Urim v'Tumim (the Rambam says that it is necessary to ask the Kohen Gadol only in the case of a Milchemes Reshus, although the Ramban in the Sefer ha'Mitzvos says a Sanhedrin is necessari in either case).
I hope things are getting clearer!
Aharon Steiner