The two things I hear people most often say to defend Israel's assault on the civilian population of Gaza are:
1. But it's in response to rocket attacks.
2. If Canada/Mexico was firing rockets into the US, we'd attack them, too.
The first is just plain silly. A man kidnaps a woman, keeps her locked in a room for years, and beats her daily. Every now and then she manages to grab a fork and tries to stab him. One day she gets the fork again so the man pulls out his gun and shoots her seven times. When the police arrive, he points to the body and the fork and says it was self defense.
The second is even more absurd. The idea that we should look to what "we" would do as evidence of some other country's right to do the same thing is laughable given that "we" invaded a country that posed no threat to us and killed the better part of a million people, and "we" operate torture prisons holding people indefinitely without charges, and "we" have a history of supporting dictatorships and military forces that kill civilians, and "we" do all sorts of other horrible things all the time. So what if the US would retaliate to rocket attacks across our borders with a wildly disproportionate response that will make our enemies more determined to kill us? That doesn't make Israel right, it makes us wrong, too.