Quote Originally Posted by Titus Marcellus Scato
Honourable conduct in ancient times (in the Greek world) was this:

A city could surrender and be treated decently provided it did so before the first siege weapon (ram or tower) reached the city wall. If it still resisted after that, then the city would be subject to a sack, massacre and enslavement.

I believe you are quoting a bit from Ceasar's "Gallic Wars" and not necessarily a widely accepted rule of war. I would have to see several quotations to convince me otherwise. Although it is a handy rule of thumb to go by, I don't think there is much proof.
The rule sounds good to me personally, but I believe we are applying our modern "ethics" to an ancient problem.