Or during the Holocaust? Or to stop the First World War? Doesn't the fact that such things happen prove that God doesn't exist, or at least that he doesn't care?
To understand the answer to this question we need to clear away some junk.
First, I went to the dentist a couple of years ago, because two of my teeth were aching. I had two toothaches at once, and it hurt. Presumably it hurt twice as much as one toothache would have done; I personally find pain a little hard to quantify. My wife, whose teeth are in apple pie order, showed me just as much sympathy as you would expect and bundled me off to the dentist, who extracted one tooth and crowned the other.
We'd had two toothaches in the house before, as it happens. Then, though, I had one toothache and my wife had the other. It hurt, but it did not hurt twice as much as one toothache; it hurt just as much as one toothache. On that occasion, nobody actually experienced two toothaches.
Two toothaches can only be added up if they happen to the same person at the same time. Pains that happen to different people can't be added up.
Similarly with deaths. However many people die, however many people are killed, nobody ever experiences more than one death. Millions died in the Battle of the Somme, but every one of them felt only one death - even my grandfather only died there once.
Second, at what level should God intervene? To stop a thousand deaths, perhaps? Fine. So nine hundred and ninety nine is not worth God's intervention? Oh, that's close enough, is it? Fine. Nine hundred and ninety eight? Nine hundred and ninety seven? Be honest, there's no number of deaths you can stop at, is there, and say God should intervene above that number but not below. So if you want God to intervene to stop a thousand deaths, you really mean that God should intervene to stop one death. But what is a death? Instantaneous death in the accident? Does it still count if the victim took a couple of minutes to die? Yes of course. A couple of weeks? Er, yes. A couple of decades? In fact, if God is to intervene to prevent a death, he has to intervene to prevent any life shortening. But that means any injury; if I cut my finger when I'm ten, it might shorten my life by - ooh - milliseconds.
Yes this is silly. But the point is not silly: the point is that there is no natural point at which God should intervene. All injuries, all pains, all disasters form a continuum, without critical points. The only boundaries are to allow all evil, or to allow no evil.
But that is exactly the choice God gave us. He originally created us for a world, for a universe in which there was no evil; where we would never know good and evil, because everything would be good. An utterly safe, utterly protected world, where God would intervene at every moment to prevent even the slightest pain. A world of pleasure and delight; a paradise of soft and voluptuous living; a garden of Eden.
But it was a garden where nothing happens; where nothing can change, because change brings choice between old and new, and therefore likes and dislikes, love and hate, pleasure and pain, good and bad. It would have been a static life. And we didn't want that. We wanted change, growth, purpose, achievement. We wanted to know change and decay, to know success and failure, to know good and evil.
God offered us paradise, and we said no.
And God said OK. He thought we were wrong, but fine, if that's what we wanted. So instead he put us in a world where we could make a real difference, where we could achieve something. He put us in a universe where there was both good and evil; change and corruption; love and hate, pleasure and pain, joy and sorrow. Where there was no safety, no protection. Where we are now, in fact.
Well, why should we complain? It's what we asked for.
But God said more. He said that the world might contain both good and evil, but he would not allow the evil to be unbounded. Evil had to be held within limits.
So firstly, he clothed us in bodies, separating us from each other, so that none of us could know more pain than our one body can take. We no longer felt each other's toothaches, only our own. Thus evil within us has a limit - a dreadful limit, admittedly, but a limit. What other limit is possible?
And secondly, having limited evil in space, he limited evil in time as well. Our bodies do not last for ever; they wear out, they are vulnerable, they decay. He gave our bodies barely enough self repairing to work at all; for most of us after a mere century practically all our body parts are unusable. For all of us, death is inevitable.
Observe the mercy of this death: for if evil increases and increases the pain and hurt in us, in the end the worst that evil can do to us is - something that was going to happen to us anyway. Because of death, evil is in the end powerless.
As on the small scale, so on the large. Evil is bound to this universe, a universe bounded in space, and bounded in time, that will have an end. Evil can never reach beyond the limits of this universe, in time and space. Thus the driving force and central focus of Evil is not another God, but a created being within this universe; a mere devil, a fallen angel, a being within this universe. Evil is bound, bounded, walled in by this universe.
But we are not bound.
We came from outside this universe, and in principle therefore we can go back outside it. We can, in theory, break through the bounds. But we are contaminated; we know both good and evil; we carry evil within our selves. If we break through, evil will break through with us, and that would take away the whole point of the setup. Only one of us who is without evil can be allowed to break the bound, to make for us a pathway out of this bounded, limited, claustrophobic universe. And even if we follow him, the evil in us must not come with us. We must first be cleaned, washed, decontaminated, if we are to be allowed to break out.
But once our bodies have worn out, what alternative is there?
So God has made such a break, by becoming one of us, breaking the boundaries of the universe, and setting up a means of decontamination, so that all of us can leave our evil behind, and get out through the gap. Although in principle this can happen at any time, once our bodies have worn out past wearing, then it has to happen. There is nowhere else for us to go.
That is what death is. Death is God's breach in the universe. Death is God's way of getting us out of here without bringing evil with us. Death is God's rescue helicopter.
But once someone we love has died, once someone we love has been rescued by God, then evil can never follow them. We know they are safe, not because we can break through the barrier and check they are safe, but because we can't. If we can't, who actually came from outside, then nothing from inside, nothing evil ever can. They are safe even from our evil.
So when we read that three thousand people died on September 11, we should not ask why God didn't intervene; the fact that three thousand people died is proof that he did intervene.
Death is God's intervention.