Quote from international law expert Gary Grant, interviewed on English Al-Jazeera:
"Any country's first duty is to protect its citizens, it's called self-defence. The question is, is that self-defence proportionate.
"Under international law, two things need to be satisfied for Israel's actions to be considered lawful. One is that they are aiming at legitimate military objects. Israel would say that they are striking at legitimate infrastructure. And of course Hamas is an organisation intent on the destruction of Israel and the Jews in Israel as part of its covenant.
"Secondly, is it proportional? ... It's not simply a case of calculating the number of Israelis that have been killed by rockets, to the number of Palestinians killed in these attacks.
"The question is, are these attacks proportionate to the military objective trying to be achieved? Israel would argue with some force that what they are trying to achieve is to prevent Hamas, an organisation set up to destroy Israel, from strockpiling the weapons, and it's doing that by destroying the infrastructure.
"If someone were to run at me, a knife-wielding lunatic, I don't have to wait for that knife to enter my heart, before I'm about to respond. I'm allowed to take pre-emptive action, in order to stop it.
"Killing civilians is tragic, but it is not against international law. It is accepted in international law, that even if you target military sites, you are going to kill civilians. If you fire rockets and missiles, that is what is going to happen.
"But in this case, it is not the deliberate targeting of civilians, it is the targeting of infrastructure and military targets. Civilians tragically do get caught up in it. It needs to be contrasted with Hamas, where every single target is at a civilian population."
I would also recommend that folks who want to learn more about how one actually has to think about ethics and laws of war have a read of Michael Walzer's "Just and Unjust Wars", as well as perusing the Geneva Convention on such things. Lawrence Freeman's "War" is also interesting reading.