A number of people have already accused Israel of breaches of international law, and even war crimes, going so far as to argue that an international tribunal should be established and assigned the task of trying suspected war criminals. If I had a dime for every imagined war crimes tribunal that never came to fruition…. The international law issues are serious but largely irrelevant, however.
1. Jus ad bellum. Gaza is not a state. It's not clear what it is—occupied territory? Mandate? If it's not a state, the UN charter does not forbid an attack. Even if it were, the UN charter would not forbid an attack as long as Israel's attack is in self-defense—which it appears to be, so far.
2. Jus in bello. Can there be a war between a state and a non-state entity? Yes! And most people think the regular laws of war apply. The laws of war forbid the deliberate killing of civilians, but so far no one has proved that Israel has deliberately killed civilians. That leaves the proportionality rule, which bars an attack that causes civilian casualties (or property damage) that are "disproportionate" to the attack. Unfortunately, no one knows what proportionality means. Can you drop a bomb on a Hamas leader that might or will kill a nearby civilian? Two civilians? Ten? A thousand? Does it matter how important the leader is? Whether he has taken refuge in a densely packed area? There are no settled answers to these questions.
3. Human rights. Ideally, the Israelis enter Gaza and arrest the rocket shooters and their leaders, try them, and convict them if they are guilty. In practice, this is impossible. Human rights law does not prohibit the use of violence when ordinary law enforcement practices are inadequate.
So where does this leave us? There is no evidence that Israel has violated international law, though perhaps it has, or will. So why so many calls for investigations and tribunals for Israelis? And what of Hamas? Why doesn't anyone want to set up a tribunal for those guys?
This brings us to the question of relevance. No one cares whether Israel has violated international law or not, except for a handful of law professors. For its more mainstream critics (as opposed to those who think it has no right to exist), Israel has acted wrongly by mistreating Palestinians over decades, and has only itself to blame for Hamas's rocket attacks. It shouldn't have squeezed Hamas in the first place; it ought to negotiate a settlement with Hamas now. On this view, Hamas is just another national liberation movement, like the African National Congress under Apartheid, which can't be blamed for using violence to overthrow its oppressors. For Israel's defenders, Israel has every right to defend itself against a bunch of violent terrorists.
The glory of the laws of war was the separation of jus ad bellum (the laws that govern the conditions under which one goes to war) and jus in bello (the laws that govern conduct during a war). Your soldiers must comply with jus in bello rules regardless of whether the war itself is legal or not, or morally justified or not. But people do not think this way. The bombing of Dresden and London were not moral equivalents in historical memory: Dresden was payback, plus it helped weaken an infinitely more odious government. As so often happens, when law and people's moral intuitions diverge, people ignore the law. When the law is international law, and so there is no independent enforcement machinery for implementing the law, governments that are sensitive to public opinion will ignore the law and try to enforce public opinion.
People around the world see the pictures of smashed houses and bleeding children and, rightly or wrongly, blame Israel. European governments worry about their restive Muslim populations. Israel has law on its side in this battle but that does not matter because it is losing the public relations war and, one way or another, Israel will find the law closing in on it because the law is what the other states say it is. In the long term, this could mean diplomatic isolation; Israel is dangerously dependent on the support of one fickle nation that is looking for a way to reestablish its international bona fides.