Just as citizens of the world momentarily became New Yorkers in September 2001, many Americans suddenly knew London this week. They recognized the destruction and the blood-splattered fear and they were outraged.

But outrage alone is inadequate. Al-Qaida is growing stronger in Afghanistan. Even after a two-year crackdown on insurgents in Saudi Arabia, al-Qaida members last week were in gun battles against Saudi forces. The London attack, following attacks in Madrid, Jakarta, Istanbul, Casablanca, Riyadh and elsewhere, belies the assertions that the terrorists were, first, stopped in Afghanistan or that the Iraq war has concentrated them within that nation’s borders.

From the beginning of the war in Iraq, the United States has tried to put a number on the insurgents there, as if counting them would lead to their defeat, and consistently underestimated. The number of terrorists is near infinite because more will join while the Iraq war staggers along and the world’s military superpower is focused there in a way that encourages attack elsewhere. The violence in London was unconscionable and inexcusable and it is not the end of these sorts of attacks.

At least 50 dead, hundreds wounded, maybe it was al-Qaida, maybe not, the point remains that any group that can attack one of the world’s financial centers when the city’s police are well aware the G-8 summit in Scotland makes Britain an inviting target can attack virtually anywhere.

To their immense credit, G-8 leaders responded calmly and forcefully. None tried to simplify the situation or exploit it, despite their very different views of the U.S.-British-led actions in Iraq. But all should take home with them the lesson that they are not safe from terrorism.

What they should do about it beyond the obvious, however, is more difficult. Certainly, it has been pointed out that the United States spends 90 percent of its transit security money on airplanes and airports when most of the commuting is done in short trips on buses and trains. There is some new federal money for these systems, but nothing like what is available for air travel.

And if the London attack does turn out to be the work of al-Qaida, reaching out across the Muslim world for help is imperative. It’s not enough to observe that the Muslim faith and the vast majority of its followers are not violent; Muslim leaders must be encouraged to speak out forcefully and more often. Multi-cultural London is the perfect place to do so.

The response in the United States Thursday was to raise the terror alert from yellow to orange in selected places, no doubt an appropriate decision and one that was made with a welcome minimum of theatrics. But, as with the expression of outrage at the London bombings, they also seemed wholly inadequate on its own.