Terms like "proportional" and "disproportionate" are being thrown around in relation to current events in Gaza, and seem to be causing some confusion.. This is just a little reminder that the word really does have a meaning in the law of war. To quote from Army Field Manual 27-10, The Law of Land Warfare:
Particularly in the circumstances referred to in the preceding paragraph, loss of life and damage to property incidental to attacks must not be excessive in relation to the concrete and direct military advantage expected to be gained. Those who plan or decide upon an attack, therefore, must take all reasonable steps to ensure not only that the objectives are identified as military objectives or defended places within the meaning of the preceding paragraph but also that these objectives may be attacked without probable losses in lives and damage to property disproportionate to the military advantage anticipated. Moreover, once a fort or defended locality has surrendered, only such further damage is permitted as is demanded by the exigencies of war, such as the removal of fortifications, demolition of military buildings, and destruction of military stores (HR, art. 23, par. (g); GC, art 53).HR refers to the Annex to Hague Convention IV, and GC to the Geneva Convention Related to the Protection of Civilian Persons in Time of War (the relevant quotes are about property damage; see Article 51 of this Additional Protocol, forbidding "indiscriminate attacks," for the application to humans, particularly subsection 5(b)).
Thus, in international law as read and taught by the U.S., an attack is "disproportionate" if the civilian deaths or property damage are out of proportion to the military objective being gained, rather than to, let's say, the damage done by the enemy beforehand or the weapons being used by the enemy. If an enemy is attacking with rifles, it is perfectly acceptable to destroy him with artillery or missiles; and you don't have to wait for the enemy to kill any of your troops or civilians before causing massive casualties among his fighting forces.
There is also a common-sense kind of "proportionality" that applies to self-defense (and not to warfare generally). This is found in the Joint Chiefs of Staff Standing Rules of Engagement (scroll to pages 16-17). It states that the force used must be limited in scope, intensity, and duration to that which is necessary to neutralize the threat. Notice again that this has nothing to do with the amount of damage the enemy has done, or the type of weapons he is using; it relates only to what is necessary to neutralize the threat (i.e., once again, the military objective). If twenty enemy ambush you, and they're lousy shots and haven't hit anyone yet, you can still kill them all. If there's one man sniping at you from a hidden place with a rifle or even a crossbow, most assuredly you can destroy him with explosives - use what you need to neutralize him, not necessarily what he's using.
These concepts and definitions make a lot of sense - I don't think anyone would like to see us modify our laws or treaties to abolish the concept. Keep them in mind in evaluating whether Israel's current response is really "disproportionate" (in a meaningful, legal sense) or not.
Update: I linked to Michael Totten as an example of "confusion" on the issue; but if he was confused at all before, he isn't now.