I was talking to a good friend recently about the war between Israel’s government and Hamas. We both agreed that Hamas is out to destroy Israel. (If you don’t agree with that, I’d like to see your comments about why. But even if you don’t agree, you might find the following discussion relevant.)

For that reason, my friend and I agreed that it’s legitimate for Israel’s government to fight Hamas.

I argued that it’s important that a war be fought justly. That’s the second of the three components of just war theory. (The three are Jus ad bellum, Jus in bello, and Jus post bellum.) Thus the title of this post. I’m focusing on the second part: fighting a war justly.

My friend argued that because Hamas is using innocent Gazans as human shields, it’s legitimate to kill those innocent people as a way of getting at the guilty. I, uncomfortably, agreed with him but wanted to find a limiting case. So I came up what I thought would be one.

“Suppose, ” I said, “that the only way to kill one Hamas fighter is to kill 10,000 innocent Gazans. Would you agree that that’s too many?”

His answered surprised me. “No,” he said, “if that’s the only way to kill that one Hamas person.”

I was momentarily speechless, which doesn’t happen to me often.

In defending his position, my friend went back to the Jus ad bellum point, the idea, which we both agreed on, that Israel’s government’s fight against Hamas is legitimate. For him, the fact that the fight was just was enough to justify killing tens of thousands of innocent people.

But, to be fair, I didn’t have a good idea of the right ratio. Is it 1 to 1, 10 to 1, 100 to 1, 1,000 to 1? I was pretty sure it was under 100 to 1, but I couldn’t say why.

So I talked to another friend who has been thinking about this. He gave me a way to think about it that was better than anything I had.

“Suppose,” he said, “that the Israeli government knows that beneath an apartment block containing 300 innocent people is a machine gun nest of 5 members of Hamas. If the government has a reasonable expectation that killing the 300 innocents along with the 5 guilty would prevent more than 300 innocent deaths that otherwise would have been carried out by the 5 guilty, then it’s legitimate.” But if it’s fewer, then it’s not.

Of course, it’s hard to know. But what probably isn’t hard to know is that those 5, if they survived, would not have killed 10,000 people each. Which would mean that my first friend’s 10,000 number was way too high.

That’s all I’ve got. What do you think?

ADDENDUM: I should add, in case there’s any doubt, that I think it’s wrong to use innocent people as human shields. That’s why the Israeli Supreme Court, in 2005, said that no longer could the Israeli military do so. There’s some evidence, though, that, even recently, the Israeli military has done so.