Several people here have dismissed the disparity between civilian deaths of Israeli civilians and of Palestinian civilians as irrelevant. These people are obviously ignorant of the international rules and laws which cover the principles of Jus Ad Bellum, Jus In Bello and Jus post bellum collectively known as Just War.
Under international law, failure to show the requirements of Jus Ad Bellum has been met prior to the beginning of a war leaves the leaders of the military adventure open to prosecution for war crimes.
The 6th requirement of Jus Ad Bellum deals specifically with the issue of proportionality. This is defined in the Stanford Encyclopaedia as
6. Proportionality. A state must, prior to initiating a war, weigh the universal goods expected to result from it, such as securing the just cause, against the universal evils expected to result, notably casualties. Only if the benefits are proportional to, or “worth”, the costs may the war action proceed. (The universal must be stressed, since often in war states only tally their own expected benefits and costs, radically discounting those accruing to the enemy and to any innocent third parties.)
http://plato.stanford.edu/entries/war/ Note the emphasis on the Universiality element. The good of your own citizens has to be weighed against the likely harm to your enemy's citizens resulting from a proposed war. The high civilian death toll in Gaza including over 400 children raises the question was this requirement fulfilled? In law a state's right of self defence is the same as an individual's. For example if a person slaps you you cannot pull out a gun and shoot him and you certainly cannot slaughter his wife, kids and neighbours. i.e. Your response must be proportionate.
Once the war has begun; Jus in Bello; further rules come into play.
In relation to the current conflict these include,
1) Obey all international laws on weapons prohibition. Under the Nuremberg Principle this applies whether or not a warring party has itself signed up to any international treaty banning or limiting the use of a particular weapon. In the case of white phosphorous, although not banned outright, there are international laws governing the circumstances and environment under which it can be used. There is prima facie evidence Israel may be in breach of those rules for allegedly using this weapon in densely populated civilian areas.
2. Discrimination and Non-Combatant Immunity. Soldiers are only allowed to use their (non-banned) weapons against those 'engaged in harm'. To deliberately target non-combatants is a war crime. Given the accusations from credible UN sources of targeting of ambulances, UN schools, the university, hospitals and even the media tower it would seem Israel certainly has a case to answer in this regard.
3) Proportionality. Again actions taken must be proportional to the military goal sought.
4) No reprisals. A reprisal is when country A violates jus in bello in war with country B. Country B then retaliates with its own violation of jus in bello, seeking to chasten A into obeying the rules.
And so arguing Hamas broke the rules of war and so therefore Israel can too is not a valid excuse under international law.
Finally there are rules regarding Jus post bellum which are, or may become, pertinent here,
1. Proportionality and Publicity. The peace settlement should be measured and reasonable, as well as publicly proclaimed. To make a settlement serve as an instrument of revenge is to make a volatile bed one may be forced to sleep in later. In general, this rules out insistence on unconditional surrender.
2. Rights Vindication. The settlement should secure those basic rights whose violation triggered the justified war. The relevant rights include human rights to life and liberty and community entitlements to territory and sovereignty. This is the main substantive goal of any decent settlement, ensuring that the war will actually have an improving affect. Respect for rights, after all, is a foundation of civilization, whether national or international. Vindicating rights, not vindictive revenge, is the order of the day.
3. Discrimination. Distinction needs to be made between the leaders, the soldiers, and the civilians in the defeated country one is negotiating with. Civilians are entitled to reasonable immunity from punitive post-war measures. This rules out sweeping socio-economic sanctions as part of post-war punishment.
4. Punishment #1. When the defeated country has been a blatant, rights-violating aggressor, proportionate punishment must be meted out. The leaders of the regime, in particular, should face fair and public international trials for war crimes.
5. Punishment #2. Soldiers also commit war crimes. Justice after war requires that such soldiers, from all sides to the conflict, likewise be held accountable to investigation and possible trial.
6. Compensation. Financial restitution may be mandated, subject to both proportionality and discrimination. A post-war poll tax on civilians is generally impermissible, and there needs to be enough resources left so that the defeated country can begin its own reconstruction. To beggar thy neighbor is to pick future fights.
7. Rehabilitation. The post-war environment provides a promising opportunity to reform decrepit institutions in an aggressor regime. Such reforms are permissible but they must be proportional to the degree of depravity in the regime. They may involve: demilitarization and disarmament; police and judicial re-training; human rights education; and even deep structural transformation towards a minimally just society governed by a legitimate regime. This is, obviously, the most controversial aspect of jus post bellum.
http://plato.stanford.edu/entries/war/
Specifically the continued imposition of the Israeli blockade of Gaza may be considered in violation of rule 3 which is enshrined in international law as the ban on collective punishment.
Ultimately the winner of this conflict may be settled in the courts.
