French President Nikolas Sarkozy has already called Israel's air raids of Hamas' terrorist facilities, "disproportionate."
Israel is bombing intensely a large number of Hamas facilities in Gaza because of Hamas' has been firing dozens of rockets per day into southern Israel. A few Israelis have died or been injured. But about 250,000 Israelis are within the range of Hamas' rockets and most of them have been forced to stay in bomb shelters for extended periods every day for several weeks. News reports from Gaza say that this week israel has killed more than 300 people, the great majority of them members of Hamas, but many civilians have also been killed or injured.
Israel's bombing can be seen as disproportionate only if the Just War Theory principle of proportionality is (wrongly) understood as meaning a tit for tat response. That is, if Hamas kills three Israelis, Israel may retaliate, but not more violently than Hamas was. This is a severe misunderstanding of what proportionality means in Just War theory.
Just War theory recognizes that a nation has the right to defend itself from aggression. Proportionality does not mean that Israel cannot fight back with more violence than Hamas is using against it. It means that its response must be proportionate to the good Israel is defending. In this case, the good Israel is defending is the right of its citizens to live and work free of the threat of sudden death from Hamas' rockets. A proportionate response for Israel is to take all measures to eliminate this threat, but not to use more violence than necessary to achieve that end. A related principle, discrimination, means that Israel must discriminate between legitimate targets and non-legitimate targets, attacking the former but not the latter.
Israel's primary obligation is to its own citizens, and it is not to limit its retaliation to the level of violence that Hamas has been using against Israel, but to use the full amount of force necessary either to remove Hamas' capability or will to do so any more.
See also Richard Cohen's 2006 column, "A Proportionate Response is Madness."