Daniel Davies at Crooked Timber on the question of proportionality that has come up often recently in discussions over the war in Gaza:
Under Protocol 1, Article 57 [of the Geneva Convention], a commander has three duties (explained very clearly in “Constraints on the Waging of War: An Introduction to International Humanitarian Law” by Frits Kalshoven and Liesbeth Zegveld):
1) to do everything feasible to verify that the chosen target is a military objective
2) to take all feasible precautions in the choices of means and methods to avoid, or in any event minimise harm to civilians and damage to civilian objects
3) to refrain from carrying out an attack if may be reasonably be expected to cause such harm or damage in a quantity which would be excessive relative to the concrete and definite military advantage anticipated.
So, under international law, for example, “minimising civilian casualties” is a basic primary requirement – it’s something you always have to do, not something you get extra brownie points for and certainly not something you can trade off against a slightly dodgy choice of target. Furthermore, “minimised” casualties could still be “excessive” relative to the concrete and definite military advantage anticipated. And international law’s clear […] too about the kind of objective that can be set against the civilian casualties; it has to be “concrete” (no messing around with intangibles like “avoiding the rocketing of New York”), “definite” (as in, with a clear chain of causation to the enemy’s ability to wage war) and “military” (no bombing objectives in order to gain political advantage).
International law’s also very clear on the subject of “negative reciprocity” – the question of whether one side’s failure to play fair releases the other side from its obligations. The answer is it doesn’t, and specifically, that even if the other side breaches its obligation to protect its civilians, by using them to surround a military objective, you don’t get to ignore the existence of the human shields; the calculation of whether the harm to noncombatants is excessive relative to the CDM advantage has to be made on the basis of the actual harm anticipated, not some wishful-thinking assessment of what it ought to be.