I know at least 3 cases where the Jewish people were saved by women, Yael, Ester, and Yehudis. Yael and Ester was done through sex, and most probably Yehudis also. It's very interesting that the Jewish people get saved through sex. On the other hand, it shows that women should only fight with their unique weapon, "sex" (Don't try it), and not with a man's weapon, "guns". There is no good reason for a girl to join the army other then "you know why".......
"A woman may not wear a male garb, and a man may not wear a female garment because it's a "toayva" whoever does this." (Devorim 22,5) The Torah would not have called it "toayva" just because of cross-dressing if not for the fact that the cross-dressing is done in order to mingle with the opposite sex and be promiscuous. (Nazir 59a, Rashi in chumash)
We also learn from here that it is forbidden for a woman to dress up in arms and go to war (Nazir ibid). "It is the way of men to make war, and not the way of women to make war" (Kiddushin 2b). "It is the way of men to conquer and not the way of women to conquer" (Yevamos 65b). And this is why Yael did not kill Sisra with a weapon but with a tent peg. (Rashi Nazir ibid) As far as female settlers carrying guns are concerned see Egros Moshe, Orach Chayim 4, end of 75.
It is very interesting that cross-dressing by itself is not considered "toayva" if not for the fact that they do this in order to mingle with each other in the army and police force and have sex. So the Torah is saying that it's all about sex. Otherwise there is no logical need for women in the army or police force if men can generally do a better job. (A much better job) The modern world likes to make fun of the Torah with all it's strict safeguards, but when you come home and you find your spouse (cop) in bed with another cop then the joke's on you.
At this point I must repeat the words of rabbi Akiva Tatz. The professor tells his class that we are no better then monkeys because we all come from monkeys. And then he goes home and finds his wife in bed with the other professor, and he screams bloody murder! Hay! what's the problem? I'll give you another banana! (Click Sep 8, 2011)