YESTERDAY'S bombings in London thankfully did not cause serious injury. What remains to be seen is whether they will damage the sense of assurance and relative political unity with which Britain so far has responded to the terrorist assault on its homeland. Londoners inspired the world two weeks ago when they quickly returned to buses and subway trains despite synchronized suicide attacks that killed 56 and wounded more than 700. But the shorthand many adopted for that day, 7/7, reflected the idea that the strikes, like those of 9/11 in the United States or 3/11 in Spain, were an exceptional event -- not one that might be repeated with chilling precision, if not the same deadly results, 14 days later. Though it's not yet known what connections may exist between the attacks, Britain yesterday contemplated the possibility of a sustained terrorist campaign focused on targets, such as urban transport, that are impossible to fully defend.

To their credit, police and municipal authorities in London pushed for another quick return to normal. Within hours most trains and buses were running again, even as the search for suspects and evidence continued. Not all returned to their routines, however: Lots of nervous commuters chose to walk or drive home. Prime Minister Tony Blair said he was resuming his schedule -- but Mr. Blair already is working on new anti-terrorism measures, including some that could curtail civil liberties or freedom of speech.

Since the July 7 attacks, British authorities have come under criticism both at home and elsewhere in Europe for permitting London -- or "Londonistan," as the critics would have it -- to become a haven for Islamic extremists preaching or inciting violence. On Wednesday Mr. Blair's government announced a series of measures to prevent known militants from entering the country, and to deport some already there. One Palestinian-born cleric considered an al Qaeda ally may soon be turned over to Jordan -- a step that would resemble the controversial "renditions" of terrorism suspects by the Bush administration and that could violate the Convention Against Torture. Mr. Blair is expected to propose further measures in the fall, including laws that might allow police to act preemptively against suspected terrorists, or arrest people who endorse extremist acts.

The government's impulse to rein in Islamic radicals is understandable, and some tightening of security may be appropriate. But the risk is that a broader crackdown would harm Britain's admirable climate of freedom without stopping attacks like the transport bombings. London's safety certainly would not be helped if Mr. Blair accepted the swelling argument of the British left -- which is that the war in Iraq somehow propelled young men of Pakistani and Jamaican ancestry to murder. The implication is that withdrawal might appease their fellow fanatics.

The hard truth is that the threat from Islamic extremism cannot be simply addressed any more than the subway and bus networks of a great city can be readily defended from exploding backpacks. Success will probably be the work of a generation and requires a mix of police work, reforms in the Middle East and in Europe's Islamic communities, and the defeat of the enemy on the battlegrounds of Afghanistan and Iraq. The best immediate response to more bombings, however inadequate it might sound, was the one Mr. Blair offered yesterday: "It's important . . . that we respond by keeping to our normal lives and doing what we want to do," he said. "To do otherwise is, in a sense, to give them the very thing that they are looking for."