And even in the times when there were milchomos mitzvah, the troops were divided into three groups of equal size: combat, support, and learning. And the women were never in combat. You’ll find no such instance in Jewish history.
In fact, 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.
Rav Soloveitchik, (certainly not an anti-Zionist), 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.