Terrorist attacks on Israel are indeed revolting and indefensible. But the bombing of densely populated areas, however accurate, is certain to cause the deaths of many innocents.
How then can it be defended? In what important way is it different from Arab murders of Israeli women and children?
One is directly deliberate. The other is accidental but unavoidable. I wouldn't say that was a specially important distinction, especially if you are a victim of it.
On the one hand, there's an important implicit point here - namely, that the moral distinction between accidentally killing civilians in pursuit of a legitimate military objective and deliberately killing civilians is much murkier in practice than in theory; that the term "unavoidable" can be employed to cover a multitude of sins; and that numbers do matter, and the more civilian deaths a military operation "unavoidably" causes, the more one should be skeptical about its justice. These are things that conservative just-war theorists, especially, would do well to keep in mind, not least because they tend to share a political coalition with thinkers and writers whose understanding of morality and war runs in a more utilitarian direction. (For instance: If you believe in just-war theory but find yourself using it to justify almost every single major policy decision the United States has ever made in wartime - as some conservatives are wont to do - then you're probably stretching your moral theory to covers things that shouldn't be covered.)
On the other hand, though, the explicit logic of Hitchens' argument has the potential to vitiate just-war theory entirely - or else reduce it to a gentlemen's agreement suited to 18th century battlefields and not much else. If highly-targeted bombing raids in densely-populated areas in the pursuit of explicitly military objectives are inherently morally illegitimate because they inevitably leads to civilian casualties, then what about house-to-house fighting in densely-populated areas? Doesn't that inevitably produce civilian casualties as well? (Answer: Yes.) Doesn't Hitchens' logic require saying, then, that any sort of significant urban military campaign is morally indistinguishable from straightforward butchery of civilians - or if a distinction exists, it's not "specially important"?
If so, he's taking just-war theory to a place so narrow, and so close to pacifism, that it ceases to have any practical application to modern warcraft. Now maybe that's where it should be taken. There's a not-unreasonable case that modern warfare by its very nature - because of military technology, urbanization, mass mobilization, the collapse of the distinction between civilians and soldiers, the rise of non-state actors, and so on and so forth - has left traditional just war theory in a state of crisis from which it's unlikely to recover. And if the theory is in crisis, then there's something to be said for Christians, in particular, withdrawing toward the more absolute presumption toward nonviolence suggested in the Gospels.
My own view, though, is that just war theory has always been in crisis, and that modernity has only heightened the contradictions - because almost all of the standards the theory sets are so malleable in practice, and so difficult to apply consistently to the complexity of war and statecraft. Consider the Catechism's definition: Who gets to define what sort of harm is "lasting, grave, and certain" enough to justify going to war? Who decides when all means of preventing conflict "have been shown to be impractical or ineffective"? Doesn't almost everybody enter a war convinced they have "serious prospects of success"? Isn't every party to a war convinced that their actions won't "produce evils and disorders graver than the evil to be eliminated"? I'm being a bit glib, obviously, since serious thinkers have drilled down on all of these questions - but the fact remains that on a case by case basis, a shared commitment to just war theory doesn't guarantee anything like a consensus on the justice of a given war or operation.
This doesn't make the theory useless by any stretch, but it's useful primarily because it provides a broad framework of restraint: If you're thinking about questions of justice, you're less likely to commit an injustice, even if no perfect consensus exists on the distinction between a licit campaign and an illicit one. But for the framework to have the desired restraining effect on statesmen and warmakers, it has to marry practicality to idealism, and strike enough of a balance between the two to make it seem applicable to real-world crises. And if it's important not to stretch the theory to justify any goal or end you seek, it's also important not to narrow it to the point where it seems so unrealistic and disconnected from the realities of war that policymakers will feel comfortable ignoring it. Which is why I find the widespread tendency to label Israel's current tactics as unjust - as opposed to labeling the war as a whole unwise, and unjust in its unwisdom - to be a somewhat troubling development: If you find yourself saying that a modern state cannot take the fight to a terrorist regime if doing so unavoidably involves civilian casualties, you're advancing a theory of jus in bello that no state can accept - and ultimately, I suspect, you're giving ammunition to the side of the debate that wants to do away with moral restraint in the struggle against terrorism entirely.