The case for Israel’s actions is self-defense. They were responding to shelling from Hamas. The best rebuttal I’ve seen to this argument comes from David Reiff in a letter to Crunchy Con:

I think you quite dramatically misstate one of the essential elements of what led up to this renewal of hostilities: the continuing Israeli blockade of Gaza throughout the truce period. I really don’t see you can on the one hand regret the support given to Hamas by the population of Gaza while at the same time ignoring one of the essential reasons for that support.
have you ever been to Gaza? I have. It is poor, but richer than many places, including Haiti in our backyard. But what makes it remarkable is its hopelessness. And in that context, believing that anything is better than the status quo may not be a sensible conclusion, but surely it is an understandable one. If Israel was not going to loosen the restrictions on access and ingress in a period of relative calm, when was she going to do so? After a final settlement? You might as well say after the Second Coming!…

None of this exculpates Hamas. They should not have abrogated the truce. But even if one leaves the question of the proportionality of the Israeli response to one side … there is the question of Israel’s right to maintain the effective sealing of Gaza ad infinitum.
To which Crunchy responds:

All the same…what choice did the Israelis have when confronted by a fanatical regime in Gaza, one dedicated to their extermination? Should Israel not have laid siege to Gaza, would Hamas have imported better and stronger missiles, and put Israel in greater danger than it is today?

The full exchange is fascinating.
UPDATE: Juan Cole, a liberal foreign policy expert, explains the interaction between the blockade and the rockets in a fascinating post charting the history of the current conflict:

Hamas pursued the tactic of sending small home-made missiles against nearby Israeli towns, mainly Sderot, emulating what Hizbullah had been doing to the Israeli colony in the occupied Shebaa Farms in 2005-2006. Israel responded primarily by squeezing the Gaza public, denying it enough food, fuel, electricity and services to function healthily, in hopes that it could be made to turn against Hamas. This punishment of the civilian population (half of which consists of children and some large proportion of which does not anyway support Hamas) is illegal in international law, and failed in its purpose. Hamas became ever more entrenched.