I wouldn't be too concerned about not being able to find any passage of Scripture that directly, explicitly, and unambiguously addresses this issue, since there are some things that were simply too absurd and outlandish for the biblical writers to have concieved without witnessing them (such as women serving in combat). However, Scripture does provide some basic principles that are quite clear about the distinctive roles of men and women, and to reject these is to reject the Word of God. Pilgrim's "impression" is exactly right: men are designed and called to be protectors and providers, and women are designed and called to be nurturers and caregivers. As Denny said, "it is natural and instinctive for the male to attempt to protect the female," and it is "natural" and "instictive" because that is the way that God made us. To try and change this through training, regardless of whether it is possible or not, is an abomination, as is training women to become killers.

When Scripture does speak of going to war, it always speaks of men doing the fighting and women staying home:

(All quotes from the TNIV)

Quote
Your wives, your children and your livestock may stay in the land that Moses gave you east of the Jordan, but all your fighting men, ready for battle, must cross over ahead of the other Israelites." (Joshua 1:14)


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If a man has recently married, he must not be sent to war or have any other duty laid on him. For one year he is to be free to stay at home and bring happiness to the wife he has married. (Deuteronomy 24:5)


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The LORD spoke to Moses in the tent of meeting in the Desert of Sinai on the first day of the second month of the second year after the Israelites came out of Egypt. He said: "Take a census of the whole Israelite community by their clans and families, listing every man by name, one by one. You and Aaron are to count according to their divisions all the men in Israel who are twenty years old or more and able to serve in the army". (Numbers 1:1-3)

The only exception I am aware of is Deborah, but as Phil Lancaster says in an excellent article, "this was a period of great decline in Israel's history when everyone did what was right in his own eyes; it is hardly an example of God's ideal." Lancaster then goes on to say that the whole point of the story is "that in times of spiritual degradation, when men are wimpy and need women to take on the roles of men and to hold their hands, God is still faithful to deliver His people...hardly a commendation of gender role reversal." If saying that the example of Deborah is a notable exception and not normative sounds like special pleading, consider what Deborah did when it was time to go into battle:


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She sent for Barak son of Abinoam from Kedesh in Naphtali and said to him, "The LORD, the God of Israel, commands you: 'Go, take with you ten thousand men of Naphtali and Zebulun and lead them up to Mount Tabor. (Judges 4:6)



I think the burden of proof is on the side of the egalitarians and feminists, whether we are talking about practical, physical, psychological, sociological, cultural, or biblical considerations. In addition to the article by Phil Lancaster noted above, there are several other very good articles and sites devoted to this topic:


Turning Women into Cannon Fodder -- Robert Knight


Women in Combat: A Time For Truth -- Al Mohler

Vision Forum on Women in the Military


Center For Military Readiness



Vicit Agnus Noster

Brad Hammond