In an earlier post (one of the first on this blog), I made the argument that the Israeli aggression against Lebanon cannot be seen as in line with just war theory, mainly due to the issue of proportionality. Now more than ever can we see how true this is, given the appalling destruction of southern Lebanon, the city of Tyre, and the southern suburbs of Beirut; with continued attacks on civilian centers and about half a million people displaced from their homes. And indeed, the United Nations envoy, Jan Egeland, called Israel's behavior a "disproportionate response" and a "violation of international humanitarian law".
And yet..... many in the United States (including the Bush administration) still professes support for everything Israel does. The following, I believe, are the main reasons for this (some of these themes were explored in greater detail in earlier posts).
First, Israeli exceptionalism. This comes in various hues. On one extreme, there are the dispensationalists-- fundamentalist "Christians" who believe Israeli strength against its neighbors will help bring about the second coming of Christ. It may sound crazy, but there are lots of people who believe this-- just look at the popularity of the Left Behind series. But while this is still probably a minority opinion overall, there are many who accept a weaker version of this theory, that the Jews have a divine right to the land of Israel, which means that their behavior cannot be judged by conventional standards. Even among the non-religious are many who believe Israel should be given a break, given the persistent anti-semitism of the past 2000 years, culminating in the holocaust.
Second, the terrorist factor. You will frequently hear people discuss how the tactics of terrorist organizations like Hezbollah and Hamas mean that a conventional response is useless. Since they don't play by the rules, they cannot be beaten by normal means. This group will sometimes argue that just war theory is a useless guide to the "global war on terror". Notice the similarity of these arguments to those in the Bush administration justifying torture.
Third, Niebuhrian realism. The protestant theologian, Reinhold Niebuhr, argued that there are no universal principles of justice that transcend all national, religious, cultural, and ideological differences. He actually used the Israeli-Palestinian conflict to make his point, arguing that it is impossible to judge the relative claims of each side by appealing to morality. Niebuhr had a huge influence on Henry Kissinger. You can see where this is going. These principles have guided US foreign policy since the second world war at least (think of the all the nasty dictators supported during the cold war) and are essential to understanding the alliance with Israel.
As you can guess by now, I have problems with all three lines of reasoning. The Catholic position, in particular, rules out the following: (i) the lex talionis, the law of retribution, or a response designed with retribution in mind (which would justify a disproportionate response from the militarily stronger party); (ii) the notion of collective guilt (clearly at play here as Israel is punishing the entire country of Lebanon for the crimes of Hezbollah, even though the majority of non-Shia have little time for Hezbollah, but are unable to disarm them); (iii) consequentialist "end-justifies-the-means" reasoning (terrorism is not a license to violate the moral law); (iv) the idea that God's present relationship with Israel includes a title to real estate (Israel is a secular state, no more and no less than any other); (v) moral relativism (there are indeed such things as universal principles of justice binding on all peoples).
At the end of the day, the only legitimate way to judge the morality of a war is through the clear just war principles, and clearly, in the current case, the response is seems highly disproportionate and thus immoral.