I'll have to admit that I've long been puzzled by the hatred most of the Western world has for Israel, but now Walter Russell Mead has enlightened me by delving into "Just War" theory.
But more moderate critics of Israel (including many Israelis) focus on jus in bello, and in particular they look at the question of proportionality. When the Palestinians flick a handful of fairly crude rockets at random across Israel, these critics say, Israel has a right to a kind of pinprick response: tit for tat. But it isn't entitled to bring the full power of its industrial grade air force and its mighty ground forces into an operation designed to crush Hamas at the cost of hundreds of civilian casualties. You can't fight slingshots with tanks.
For many people around the world, this seems patently obvious: Israel has a right to respond to attacks from Hamas but it doesn't have an unlimited right to respond to limited attacks with unlimited force. Israeli blindness to this obvious moral principle strikes many observers as evidence of hardheartedness and national moral decline, and colors their perceptions of many other Israeli policies.
The whole jus in bello argument sails right over the heads of most Americans. The proportionality concept never went over that big here. Many Americans are instinctive Clausewitzians; Clausewitz argued that efforts to make war less cruel end up making it worse, and a lot of Americans agree. [UPDATED NOTE: Many Americans consider the classic concept of proportionality -- that the violence used must be proportional to the end sought -- as meaningless when responding to attacks on the lives of citizens because the protection of citizens from armed and planned attacks is of enough importance to justify any steps taken to ensure that the attacks end.]
Just War theory really makes the most sense to me in the context of disagreements between individuals. Historically that's what wars have been: one aristocrat fighting against another for personal reasons. Among modern democracies though that paradigm doesn't hold. "You killed five of my peasants so I'm going to kill five of yours" is fine if peasants only have value as pieces of property, but once you start to see those peasants as citizens with inherent value of their own then proportionality goes out the window. Every citizen is immeasurably valuable on his own merit and deserves to be protected, not because of his value to his lord but because of his value to himself.