If a principle of proportionality is going to hold , you have to exhaust all possibilities. Let's put it into a simple case, if a man with a knife is threatening a woman and the woman has a gun, should she use the gun? Does a gun have the proportional force to a knife, or a woman to a man? How should we combine all those factors to make the process proportional?
This attack-defense process is not a fully controlled process by either parties. And estimates and expectations can not be precise, so the idea of proportion become operationally impossible.
Then proportionality should be a case-by-case reference, when you can reasonably exhaust all possibilities.
But with war issues, proportionality is just not applicable. Because war is about to win, more force will result in quicker victory and less cost, proportionality will only make the war last and even higher costs.
We should talk about proportion in a larger picture than just an isolated moment, in fact Israel has tried disproportionate response the other way around, they withdrew from SL but only ended up with thousands of rockets and missiles poking at nose, is this stock piling of rockets a proportional react to the withdraw?
What if the guy behind ten children is aiming at 15 children? What if the orphanage is full of rockets underground, which is potentially capable of killing hundreds? Does any conflict justify the use of a nuke? What if the guy become a suicide bomber later? You never know, will you? If no nuke were dropped, would American lose the war?
I think the "proportionate response" will not hold, because tension cumulate, a full scale by all means is the final stage, just a question of how to get there. Israel has been proportionate for a while but it does not solve anything, with a nuke threat coming, maybe they decide better try the opposite. With someone full of religious nonsense, it is hard to find what is proportionate, because a picture may piss off and start to kill. It is difficult to measure the damage when comes to religious issues, and if you can not measure the damage, how can you know what is proportional to it? If Israel is only allowed two for two, no matter it is humane or bridges or anything else, Israel will be outnumbered by far, Will anyone accept a rule of game which will only lead them to defeat?