International law is not cut and dry, but there are a number of precedents that have been established in past conflicts. International law defends, as a minimum, the right of a state to act to protect against threats to its political independence or territorial integrity [Brunson MacChesney, "Some Comments on the `Quarantine' of Cuba," The American Journal of International Law, 57 (No. 3, 1963), 595.] and to act when the imminence of attack was of such a high degree that a nonviolent resolution of a dispute was precluded.[McDougal, "The Soviet-Cuban Quarantine and Self-Defense," p. 598]
International law also insists that the recognized purpose of self-defense is to deter aggression and to protect the interests of the state. [James Francis Gravelle, "The Falkland (Malvinas) Islands: An International Law Analysis of the Dispute Between Argentina and Great Britain." Military Law Review, 107 (1985), 56. ]
Its goal is preventive in nature and not retributive. [D. W. Bowett, Self-Defense In International Law (New York: Fredrick A. Praeger, 1958), p. 20]
We can argue until the cows come home if Israel's military and political strategy will achieve this goal; only time will tell. But, Israel is in her right to respond to attacks against her citizens and territory.
The problem is that the nature of warfare is evolving as we speak. International and human rights laws concerning warfare that were written post-Second World War, in the aftermath of the devastation of Europe and America's bombing of Japan fail to address the sort of conflicts Israel now faces; there was little consideration, or even knowledge of the kind of warfare that exists today between state and non-state entities. It is a maddening fact that so-called political groups like Hamas cannot be tried for blatant violations of human rights laws; even the Palestinian Authority could not be held responsible, as they do not as of yet represent a state (and there are many that believe it was Arafat's intent to prevent statehood in order to distance the group from accountability).
To those who suggest Israel does not regard international law, I would say just the opposite is true. In fact, the IDF even has a legal department that considers and advises on all aspects of law and warfare. It would be more correct to say that all modern, western states are aware that international laws concerning both war and human rights are generally ambiguous and can be pushed and stretched as the need demands. In Gaza, however, Israel went above and beyond what any other state would do to protect civilians. This is an interesting interview with a British former military officer on the subject, and a British soldier with the Intelligence Corps in Iraq and Afghanistan comments here.
The IDF is actually one of the few western armies that has a moral code. Here's a relevant article on the subject. I would agree with this quote:
"The Israeli Defence Forces’ ethical standards are different from, and in some ways higher than, the British army’s, says Paul Robinson, but in the end the question is not whether IDF actions are moral, but whether they are wise." This is, of course, a whole other issue.
That said, I think it's generally true that no party in this conflict is seeking 'peace' as such. Israel demands security and recognition; the Palestinians demand justice and compensation; and the Arab world demands a restoration of honour and lost lands. Peace will be the outcome of these demands being met.
Therefore, all parties are equally responsible for achieving peace. The problem with placing the onus on any one party is that it is, in the end, counterproductive. Any unilateral moves made by, for example, Israel, such as the return of Gaza to Palestinian rule, actually sets back peace in that it reduces Israel's sense of security. And as we saw, as soon as rockets and mortars began flying over the border, Israeli lives and well being were also threatened.
There is also a problem with the notion of moral equivalency in these kinds of conflicts. There are acts which must be condemned; blindly criticizing the use of force when it is clearly a response to acts of terror, and then suggesting that both sides are equally responsible is simplistic. And this conflict is anything but simple.
I cannot describe the problem any better than this:
"Sometimes the desire to form a balanced judgement of the conflict, and not to attribute responsibility for the conflict largely to one side, a generally positive aspiration, may prove a pitfall and distort the picture...Symmetries in objectives, modes of action, and interrelationships of two sides in the conflict must be examined carefully. It is sometimes the very desire to adopt the stance of a neutral judge that leads the observer to close his eyes to inconvenient patterns of behaviour on the part of one side or the other in the conflict or at least to regard them benignly. Adopting a neutral stand is liable to intoxicate the observer with a euphoria of self-righteousness; he will derive great satisfaction from his rectitude and from his capacity to transcend issues and view them from lofty heights. His awareness of affinity with one side of the conflict may inculcate in him a tendency towards ostentatious neutrality, leading him to tip the scales and do violence to the facts." (Y. Harkabi: Nuclear War and Nuclear Peace, Israel Program for Scientific Translations, 1966, p. 270).