What is the biblical basis or precedent for restricting women from serving in war? In the United States, the Secretary of Defense just lifted the ban on women in combat.  Many Christians seem to oppose this idea and believe that women should not be put in combat.

What is the biblical basis or precedent for this position?
Just because there is no explicit prohibition on a thing does not mean that the Bible &quot;endorses it.&quot;
Unwritten Expectations
There is, for example, to the best of my knowledge, no biblical prohibition cannibalism. Possibly it wouldn't fall under definition of kosher, but it is at best ambiguous! In spite of this, the author of the text rightfully expects that the reader is revolted when, (in 2 Kings 6:24-33), during a siege, two women concoct a plan to eat their children.  When the second woman reneges and the first brings it before the King, the King rends his garments to show how he too understands how appalling this situation is. That, even in the absence, of an explicit prohibition in the Torah.
Likewise, the idea of women in combat would have simply been so outrageous as to not need explicit mention.   Women simply did not fight* or if they did, something was wrong.
This is not to say that every Jewish custom is somehow holy - but &quot;unthinkable&quot; things don't need explicit mention in a contract between two parties - which is what the Old Testament laws were.
*But back to that &quot;women didn't fight.&quot;  There are two notable exceptions to that rule, but both fall into the category of 'the exception that proves the rule':
1. Judith
In the apocraphyal book of Judith, for example, Judith uses sex and cunning to murder Holofernes, the invading general.  Scholars all agree, this fictional tale was written, in many ways, for its shock value. Like David defeating Goliath, this story simply assumes the incongruity of the weak defeating the strong in combat, leaving the reader to marvel at the tables being so turned.
2. Deborah and Barak
Within canonical Scripture, however, the most important story to remember is that of Deborah and Barak (no relation to the current President). Here, like in most of Judges, the Children of Israel have been beset by the enemy for their wickedness, and must now rise up and defeat the enemy. The chosen leader, however, is too reluctant.  (Yeah, it really is a different Barak!) He demands that Deborah - the woman God sent to encourage Barak, must accompany him and assist in leading.
Now, on the face of it, you might say: Hey! See - a Woman fighting in combat!  The precendent is not only absent, but wrong!
But here's the rub - God (and Deborah) bring judgement against Barak for his lack of cohones. The author writes:

8 Barak said to her, “If you go with me, I will go; but if you don’t go with me, I won’t go.”
9 “Certainly I will go with you,” said Deborah. “But because of the course you are taking, the honor will not be yours, for the Lord will deliver Sisera into the hands of a woman.” So Deborah went with Barak to Kedesh.

That Barak would have no honor from this battle is quite the indictment.  The assumption that a woman should not be in battle is so implicit as to be laughable.
Women encourage, Men fight. That's the model.  Are there exceptions? TO be sure - but the Bible is clear its not the model to follow.