Define "Proportional"

Israel is being condemned by some for responding to the capture of three soldiers and a few more deaths with an offensive in Lebanon and Gaza. While I have some doubts about whether Israel is responding wisely (I just don't know what their ultimate objective is), I do reject the "proportional" complaint.

Don't our enemies count on "proportional" responses to keep fighting? After all, they boast that we love life and they love death. They count on killing two or two thousand of us and then we just kill a few of them or perhaps a few thousand in response. They can take such exchanges far longer than we can in their thinking. In this sense "proportionality" is just fighting on their terms. It is not fighting to win--it is fighting until the enemy wins.

So doesn't the whole concept of "proportional" have to consider the other side's pain threshold and not just count bombs dropped versus suicide vests detonated? Or just counting the dead on each side?

As far as I'm concerned, if an enemy attacks, a "proportional" response means crushing the enemy in response to their attacks. If they still feel like fighting after our retaliation, we did not by definition retaliate enough.