Proportionality, Volokhs, and Lebanon

The Volokh Conspiracy presumably decided they needed a balance against intellectual heft, and added David Bernstein to the group blog (Todd Zywicki wasn't posting enough). Bernstein writes absolutely predictable posts about the Arab-Israeli conflict, but he almost wrote an interesting post describing a moral question before screwing it up.

He uses a hypothetical to ask what level of enemy civilian casualties should change your military tactics to minimize those civilian casualties, when the changed tactics endanger your own soldiers. That's an interesting question, but then the screwup:

"Assume also the the target is sufficiently important that even one hundred deaths on the other side would be considered "proportionate" under whatever standard you use for such things." (Bernstein's hypo imagined the change in tactics would cost two soldiers on the friendly side). If you add that proviso, there's no reason to ask the question - you've already reached the conclusion.

Anyway, I don't know what's proportionate under the laws of war, and I highly doubt it's defined, but it's the standard a non-pacifist has to go by. As someone who started off uncharacteristically favoring the Israeli position in the latest blowup, I'm seeing that it's also the standard that Israel is demolishing.

To answer Bernstein's question, my intuition is that somewhere past ten civilian deaths to save two of your soldiers, then you have to draw the line - anything more makes a mockery of "proportionate".