ONCE AGAIN a peaceful morning in a Western metropolis has been interrupted by a bloody and unexpected terrorist attack. Four bombs exploded on three London subway trains and a double-decker bus in the space of 56 minutes yesterday morning, killing at least 37 people and injuring some 700 others. The attack, widely if not conclusively attributed to al Qaeda or its allies, did not employ the sophisticated weapons that the terrorists are suspected of seeking, and the casualties and disruption were lower than those of last year's train bombings in Madrid. The attackers nevertheless managed to take Britain's police and security services by surprise and set off nearly simultaneous explosions aimed, as London's mayor put it, "at ordinary working-class Londoners, black and white, Muslim and Christians, Hindu and Jew, young and old -- indiscriminate slaughter."

The bombings proved that the threat of large terrorist attacks remains very real in the democracies allied in combating Islamic extremism. But they also probably ensured that that alliance will be strengthened.

As happened last year in Madrid, the terrorism was timed to coincide with a major political event: in this case, the summit meeting of the Group of Eight industrial nations that opened Wednesday in Gleneagles, Scotland. But if Spanish voters reacted to an attack just before their election day by choosing a prime minister committed to withdrawing from Iraq, the London bombings caused democratic leaders who had been quarreling over such issues as foreign aid, global warming and the European budget to quickly unite. German Chancellor Gerhard Schroeder called the bombings "perfidious attacks." French President Jacques Chirac declared that "this scorn for human life is something we must fight with ever greater firmness." Despite their differences, including those over the invasion of Iraq, the G-8 nations have engaged in unprecedented cooperation in areas including intelligence and financial monitoring during the past four years; if any of their governments wondered before yesterday if such exceptional efforts were still necessary, they no longer will. President Bush said the resolve of other summit partners in combating terrorism "is as strong as my resolve. We will not yield to these people."

Neither, to all appearances, will the people of London. Though mass transit was halted and some streets were blocked, they neither panicked nor fled in the face of the terrorism. People went to their jobs on foot if necessary, and the pubs were full. Awarded the 2012 Olympics just the day before, the British capital demonstrated that it, like all other large Western cities, is vulnerable to terrorists, but also that it can handle an emergency. Most who spoke out were as defiant of the extremists, and as determined not to be moved, as Prime Minister Tony Blair -- or left-wing Mayor Ken Livingstone.

"They seek to turn Londoners against each other, and Londoners will not be divided by this cowardly attack," Livingstone said. To "those who came to London to claim lives," the mayor had a message: "Whatever you do, how many you kill, you will fail." Hear, hear.