If you're even slightly claustrophobic, a ride in the London Underground can be unnerving. It's the oldest subway in the world; some of the stations on the venerable Circle Line were opened in 1863, when Queen Victoria was still deep in mourning over the recent death of her beloved Prince Albert. The trains are cramped and stifling, and the pitch-black walls of the ancient tunnels seem to close in on you, almost like a tomb.

When I lived in London in the early 1990s, sometimes a train I was on would suddenly stop. There we would sit, trapped in one of those dark tunnels, for 10 or 15 or even 20 minutes. Usually the reason was a bomb scare -- London in those years had to be vigilant because of the Irish Republican Army. It always turned out to be something like an innocent gym bag left on a platform. The train would creak back to life, and you could feel the anxiety subsiding in the crowded car as shallow breathing gave way to a deep, collective sigh of relief.

So the terror yesterday morning on the Circle Line must have been unimaginable. One survivor interviewed on television described smoke filling his train as passengers debated whether to stay put and suffocate or try to break out of the train and risk electrocution. That so many people evacuated one of the world's busiest transit systems without mass panic is a tribute to British stoicism and resolve.

You could argue that it was also a tribute to Britain's experience in living with terrorism, but London hasn't seen anything like this in a very long time. The IRA targeted its bombings and almost always phoned police with a warning beforehand. The bombings yesterday -- from now on, I suppose, the day will be known as 7/7 -- were indiscriminate. Their one unambiguous message: We kill you.

London was attacked just 24 hours after the city had been named to host the 2012 Summer Olympics, but that is probably coincidental; since four coordinated bombings would take so much planning, it seems more likely that the attack was timed to coincide with the Group of Eight meeting in Scotland. Still, the irony was hard to avoid. One day, crowds cheering in Trafalgar Square; the next, bloodied victims staggering into the sunlight.

We're all bloodied, staggering victims right now -- bloodied but unbowed. When you're hit like this, you do what Londoners did during the Blitz: You pick yourself up, bury the dead and hit back.

But how? Who? Where?

After Sept. 11 we had a clear target. We went into Afghanistan, swept aside the Taliban and launched an all-out hunt for Osama bin Laden. We cornered him and almost caught him, but before we could finish the job we were diverted into an invasion of Iraq. At that crucial point, didn't we lose sight of the real enemy? Didn't the United States and Britain look away from the man who had shown his determination -- and his ability -- to wreck Western economies and take Western lives?

With London still counting the dead and injured -- the terrorists attacked a double-decker bus in addition to three trains -- this isn't the moment to debate the Iraq war. But it's also not the moment to avoid facing inconvenient facts. Iraq is now filling the role that Afghanistan once filled, as an incubator and training ground for terrorists, most of whom are motivated by a warped sense of Islamic fundamentalism. Al Qaeda has been damaged, but bin Laden is still alive, we believe. Unless it turns out that yesterday's bombings have some totally unrelated genesis, it appears he is still able to mount or inspire deadly attacks.

Before the subway bombings, Muslim immigration was already a growing issue throughout Europe. Now, if al Qaeda-inspired terrorists are found to have mounted the London bombings, European leaders will be challenged to keep suspicion from growing into all-out xenophobia. Somehow we have to wage this fight in a way that doesn't turn it into some kind of clash of civilizations. Somehow we have to fight back in a way that doesn't create more terrorists than we eliminate. If we don't, simple arithmetic becomes the enemy.

"Live your lives," our leaders say. And, of course, that's all we can do. Just as we continue to work in skyscrapers and fly on airplanes, we'll continue to descend into those dark, endless, tomblike tunnels beneath the streets of London, and the more commodious tunnels beneath other cities.

But each of us will have to hope against hope that today, this morning, this evening, the train doesn't stop suddenly between stations.