Here's a cheery thought: The more we learn about the deadly bombings in the London Underground, the more it seems there's nothing anyone could have done to prevent them.

I should say almost nothing, since there were a couple of long-shot chances. The powerful explosives used to blow up three subway trains and a double-decker bus on 7/7 wouldn't have been in stock at the local hardware store; authorities might have intercepted this material before it reached the bombers' backpacks. Also, if police are right in assuming the suicide bombers were just fodder, whoever planned this atrocity -- possibly a hardened terrorist, possibly from overseas -- might have been nabbed by a sharp-eyed immigration officer.

The likelihood, though, is that only luck was going to uncover this plot, and luck isn't a plan.

So welcome to the new reality of Western life. If you understand probabilities, you won't lose sleep; you'll know that unless al Qaeda somehow gets its hands on nuclear or biological weapons, almost every single one of us will be just fine. But you'll also know that some of us, a relative few, will not. That is the unambiguous promise of the London attacks.

Was there a way to "harden" the Underground against bombers? No, not the Underground or the New York subway or the Washington Metro or any other major transit system. Imagine trying to put metal detectors on subway platforms. Imagine wand-wielding security officers at bus stops, asking riders to take off their shoes.

Michael Chertoff, the secretary of homeland security, told Post reporters and editors Wednesday that his department could never lose its focus on the fact that a nuclear or biological attack would be "an order of magnitude" worse than any subway bombing. Americans need to build a "resiliency factor" into their concept of dealing with terrorism, he said.

I understand that. What I don't understand -- and I believe that in order to fight terrorism without quarter and crush it, we'd better try to understand -- is what could make someone do what those four men did.

From what we know or surmise, three of the four suicide bombers were unremarkable young men from the unremarkable city of Leeds in the north of England. They were of Pakistani descent, ordinary citizens of today's multiethnic Britain. Shehzad Tanweer, according to reports in the British press, was an affable jock whose father owns a neighborhood fish-and-chips shop. Mohammed Sidique Khan was a teaching assistant who gently mentored at-risk kids from troubled homes. Hasib Hussain, a somewhat aimless teenager, loved playing soccer and cricket.

Police were slower to identify the fourth bomber, but press reports say he was of Jamaican descent and did not live in Leeds.

What's so chilling is that you can't really call these men a terrorist "sleeper cell." They were home-grown: the maniacs next door.

Something changed these ordinary men, and I'd sure like to know what that was. In retrospect, friends told reporters, they had become more religious in recent months. The London newspaper the Independent quoted an acquaintance who said the three Leeds bombers had been seen at a radical mosque. British Prime Minister Tony Blair has pledged to take action against radical clerics who preach "a perverted and poisonous misinterpretation" of Islam. But most of Britain's 1.6 million Muslims are immune to jihadist poison. What made these men so receptive?

In a courtroom in Alexandria on Wednesday, a federal judge sentenced Islamic scholar Ali Al-Timimi to life in prison for inciting young Muslims to join the Taliban and fight against U.S. forces in Afghanistan. It's always difficult in a free society to criminalize speech. Let's assume that in this instance the punishment is justified. Will putting Timimi in prison do any good? Or does hateful jihadist exhortation simply give oxygen to a fire that is already smoldering?

A couple of days after the bombings, Londoners went right back to riding the subways and buses. I stand to applaud them. That was a stirring act of defiance, a message to terrorists that We Are Not Afraid. But defiance isn't quite the same thing as victory.

Since the bombings, there have been more than 100 reported attacks against Pakistanis in Britain, including one killing. The hard-right, anti-immigrant British National Party has already used images of the bombed-out London bus in its campaign advertising. But revenge surely isn't the same thing as victory.

To win this war we need to understand the enemy. Ignorance isn't the same thing as victory, either.