The Chinuch writes that the prohibition of showing fear in battle and the other mitzvos that apply to battle (e.g. yateid al azeinecha) are incumbent upon men and not women, as women do not go out to battle. Since these mitzvos apply to milchamos mitzvah, wars against Amalek and the Seven Nations of Canaan, as well as milchamos reshus, the Minchas Chinuch asks why women are exempt - the Mishna tells us that even a kallah must go out from her chuppah to fight in a milchemes mitzvah.
The Rambam (Melachim 7:1) writes that the Mashuach Milchama read the parsha warning not to fear and granting dispensation from battle for a newlywed, someone who had just built a house, or someone who planted a vineyard before all battles, both milchamos mitzvah and reshus. Ra’avad, however, holds that the list of dispensations are not read before a milchemes mitzvah. No one is exempted from serving in battle during a milchemes mitzvah - again, as the Mishna writes, even a kallah goes out to fight (how about a kallahmagazine article on warfare?) – so there is no need to announce the normal dispensations. Why does the Rambam disagree?
Rav Soloveitchik explained that aside from the mitzvah of waging war (chovas tzibur) against Amalek and the Seven Nations, there is an obligation (chovas yachid) incumbent upon each Jew to kill members of these nations (Melachim 7:4). The participation of women in battle is not based on the communal obligation of warfare, but on their mitzvah as individuals to fight against these evil nations. This does not effect their exemption from mitzvos that relate to the communal act of war. The same reasoning applies to the reading of the Mashuach Milchama. There still exists a dispensation from the communal act of war for those people normally exempted, but they must participate in some way to fulfill their obligation as individuals to assist the destruction of evil.