McMahan's main claim is that too few deaths of Israeli civilians will be prevented by Israel's campaign, considering the cost in Palestinian civilian deaths; this is largely because Israel's anti-missile system is already preventing Palestinian missiles from killing civilians. There's therefore a problem of disproportionality.
As plausible as that conclusion is, it's strikingly unpersuasive how McMahan gets there. He proposes to start with ordinary, personal self defense:
Suppose your life is threatened by a culpable aggressor but your only effective means of defence will kill an innocent bystander as a side effect. Most philosophers believe that it would be impermissible to save yourself at the cost of killing this innocent person.Is that what "most philosophers" believe? I'm not sure, but I think McMahan is surely wrong about why they believe it, if they do. Here's his proposed explanation:
This is mainly because the moral reason not to kill a person is stronger than the reason to save a person. It is therefore generally impermissible to kill one person to save another, even if the killing would be unintended.This can't be the whole story about bystander deaths, because this story also implies that it's wrong to kill an attacker in self defense. Suppose someone is aiming a missile at my house and I (an innocent civilian) can't escape. All I can do to save myself is fire back, killing the attacker. This would be a case of saving a person (namely me) by killing another. Should I stop and think "the moral reason not to kill a person is stronger than the reason to save a person"? No, so the ''better not to kill than to save" principle, so simply stated, isn't tenable and probably also isn't what most philosophers believe.
McMahan actually aims to discuss proportionality as it pertains to unintentionally killing bystanders, not attackers, in a self defense situation, but it's interesting first to notice how little we care about proportionality when it comes to attackers. Suppose it takes 10 people to operate my attackers' rocket launcher, while mine is a solo model. I don't have any reluctance to say I can fire back and kill 10 to save just one (me). If it takes 100 to operate their rocket launcher, I may kill 100 to save myself. People trying to kill me can't undermine my right to fight back simply by piling on!
McMahan asks how many bystanders you may unintentionally kill, if you're killing an attacker in self defense. His answer is simple: not even one innocent bystander may be killed to save yourself. That follows from the "better not to kill than to save" principle. Sticking with that analysis, his objection to Israel's campaign will be that there are more bystanders being killed than civilian deaths being prevented. But McMahan takes on board what most people think: that there is some special prerogative to preserve yourself, permitting one bystander to be sacrificed to save yourself from an attacker. How about two? Maybe, he says, but not three! From that standpoint, McMahan judges that Israelis should be killing at most 2 Palestinian civilians to save 1 Israeli civilian. And then the problem is that the actual ratio is far from that: there have been over 1800 mostly civilian deaths in Gaza so far and perhaps only a handful of Israeli civilian deaths have been prevented, considering that Israel does have an anti-missile defense system already protecting civilians.
These numbers--1 or 2 bystanders may be unintentionally killed to save yourself, but not 3--have nothing but intuitiveness going for them, but do they have even that? Again, 10 guys are operating a rocket launcher and I'm on the verge of firing back with my solo launcher to save myself. Let's suppose (contrary to the simple "better not to kill than to save" principle) that I'm entitled to kill the 10 guys. So the issue is only about bystanders. It's part of the 10 guys' strategy that they will keep some children close by while firing at me, knowing I'll be reluctant to fire back under those circumstances. McMahan gives me, at the outer limit, the right to cause two bystanders deaths, but not three. If I happen to have my own child in my house with me, then to save the two of us, I get, at the very most, four bystander deaths. If there are five or more kids in the attacker compound, morality requires me to succumb to the attack, along with my two kids. Obviously, we're being offered just an illusion of precision here. There really isn't an answer to how many bystander deaths are tolerable. The real intuition most of us have is that there shouldn't be gross disproportionality!
But perhaps we have other thoughts about this as well. It seems different that the attackers are deliberately using the children as innocent shields, compared to a situation in which, by sheer accident, there happen to be children nearby. We might be inclined to say, in the first case, that if I exercise my right to preserve myself, the deaths of the children will on my attackers' conscience, so to speak. In the second case, they've got to be on my conscience. We care not just who causes which deaths, but who is to blame for which deaths. Assessing causality is just part of assessing blame.
McMahan highlights something I do find morally perplexing: Israel does have an anti-missile defense system already in place, so the number of Israeli civilians saved by the Gaza campaign cannot be great. That does make the number of Palestinian civilians killed seem troubling. But you can say why without even mentioning proportionality. It's a truism that if you can kill fewer rather than more to achieve the very same goal, you should. Killing the larger number makes you guilty of gratuitous killing. Granted, we do have some moral intuitions about proportionality too, but they're very imprecise (contrary to McMahan's pretend moral math). There's nothing imprecise about the prohibition against gratuitous killing.