Double Effect and Proportionality
I have to concur with Matt; the destruction of Lebanese infrastructure cannot be seen as a proportional response to the kidnapping of Israeli soldiers. I like Israeli soldiers as much as the next guy, and I hope that the two kidnapped by Hezbollah will be returned safely, but destroying the transportation network of Lebanon (not even the aggressor) is hardly a reasonable action to take, either by the doctrine of double effect of the principle of proportionality.
Chait notes that Hezbollah started this, and that’s important as far as it goes, but it doesn’t go very far. The jus in bello restrictions apply regardless of whether the other side is abiding by them, and regardless of who started the conflict. They apply especially to the stronger side; Israel cannot claim to be in a situation of extreme duress. That said, and I again agree with Matt, firing on Hezbollah artillery sites even when in civilian areas would seem to be reasonable and legitimate under double effect.
Then again, I’m no expert in Just War theory.