The beginning of the current round of hostilities should be dated from Israeli actions in a Gaza tunnel that resulted in the deaths of six Hamas militants. Although Israel denied responsibility, it admitted that it had carried out operations in the area that plausibly led to the incident. This was Monday night, and Hamas retaliated with rockets. A time line can be found here; thus started the current round of hostilities.
If we assume that Israel had a hand, directly or indirectly, in the deaths of the Hamas militants, that would be a serious breach of the cease fire, and, assuming that no other effective recourse was open to it, it would be permissible for Hamas to retaliate militarily, at least according to just-war theory. It would be hard to argue that just war theory can only apply to state actors, and that Hamas, a “terrorist organization”, is not a state actor – because Israel considers Hamas to be the governing body of Gaza, and hence accords to it some of the responsibilities of a state actor. (This would be different, for example, were Hamas to be operating in the West Bank in areas under Israel’s direct control.)
The only principle I can think of that would counter this is that the Palestinians do not have a right to self-defense. I can’t think of any convincing argument for this.
So having established that the Palestinians in Gaza have a right to self-defense, and hence, to retaliate (jus ad bellum), the question would then turn to the morality of their conduct of military operations (jus in bello) And here they are on much weaker ground, since their conduct consists solely in indiscriminate rocket firing towards civilian targets.
Yet this is where the question gets interesting: were the Palestinians to have a serious weapons capability, and were they then to fire indiscriminately then it is clear that their conduct of the operations would violate just war principles. The same would certainly be true if Hamas turned to suicide bombing. But, paradoxically, the primitiveness of the rockets, together with the Iron Dome defense, and Israel’s early warning system, has so far guaranteed few if any civilian casualties. True, there is some damage, and certainly there is the inconvenience of having to go to shelters, and the rockets make people anxious, despite the heavy odds against them being hurt. It may be irrational to buy a lottery ticket, given the odds, but people do all the time. Still, the fact that the odds that a given person will be actually hurt by a Kassam rocket are extremely low, virtually nil, given early warning systems and Iron Dome, plus the Hamas’ militants knowledge of this fact, suggest that if this method of conduct is not just (and I don’t think it is), it is a lot less unjust than the Israeli response, which has claimed to date over a hundred lives, most of whom are civilians. It is certainly no exaggeration to say that the Israeli public’s suffering does not compare to that of the Gazan people on any metric; the Israelis have a highly effective defense against primitive weapons, whereas the Gazans have no defense at all against highly sophisticated and deadly weapons.
So both sides are committing war crimes, but those of Hamas pales in comparison to those of Israel. And this, of course, without reference to the fact that Israel broke the cease fire.