For several years, the British prime minister and senior officials of his police force had been warning that a terrorist attack on London was "inevitable."
On Thursday, the "inevitable" struck, killing at least 33 people and wounding 360. Sixteen months after similar bombs killed 191 commuters in Madrid, London appeared also to be paying the price for Britain's alliance with the United States in Iraq.
"London is the capital of one of the oldest imperial powers, and of one of the powers that invaded Iraq," says Joachim Krause, a terrorism expert at Kiel University in Germany. "There seem to be many reasons to target it."
An unverified claim of responsibility for the four deadly blasts from the "Secret Organization Group of Al Qaeda of Jihad Organization in Europe" called the attack "revenge against the British Zionist crusader government in retaliation for the massacres Britain is committing in Iraq and Afghanistan."
Mr. Blair responded that "our determination to defend our values and our way of life is greater than their determination to cause death and destruction to innocent people in a desire to impose extremism on the world."
"The international intelligence community has been talking for some time about potential blowback from the Iraqi conflict," says Magnus Ranstorp, a terrorism expert at St. Andrews University in Scotland.
"The French say they know of about 100 of their people who have traveled to Iraq, and 70 or 80 British nationals are thought to have been there," he adds.
The claim of responsibility, posted on a jihadist website, warned "the governments of Denmark and Italy and all the crusader governments that they will be punished in the same way if they do not withdraw their troops from Iraq and Afghanistan."
But that does not mean other European governments can breathe any more easily, says a senior French official recently involved in antiterror measures who spoke on condition of anonymity. "No political position protects you from these sorts of people," he warns. "To them we are all crusaders, and there is an infinite world of threats. No country can say they face no threat."
Though the group saying it had carried out the London blasts had not previously been heard of, terrorist experts suggest they might be the work of an ad hoc grouping of Islamist jihadis such as perpetrated the Madrid bombings.
In that case, recalls Dr. Ranstorp, "a complex constellation of groups coalesced around one individual."
"It's very similar to the attacks carried out by North African extremist groups like the one in Madrid," says Evan Kohlmann, author of "Al Qaeda's Jihad in Europe." "As the number of dead rises, it becomes more and more likely that it was carried out by an existing Islamic extremist group" with planning experience and training.
The group appears to have taken into account not only the fact that world attention would be focused on Britain as G-8 leaders met for their annual summit, but also that British police and intelligence agencies had turned their attention to the summit site in Scotland, and away from London.
European governments have taken a number of steps since 9/11 to defend themselves better against terrorist attacks, but all of them have focused on improving police and intelligence capabilities rather than seeing their work as a "war on terror," as Washington has declared.
The European Union has named an "antiterror czar," Guy de Vries, who is paying as much attention to preventive political and social measures to stop the next generation of European Muslims from joining the jihadist cause as he is to current crises.
The EU has also established a "situation center" in which intelligence and police officials from different nations monitor different antiterrorist operations and share information. In Paris, US Central Intelligence Agency officers work with European and other counterparts at a secret center known as "Alliance Base" to plan antiterrorist operations, according to a recent report in The Washington Post.
But this is a far cry from genuine continental cooperation, says the French official. "Intelligence services get information according to their own systems of alliances, and they exploit it for their own interests," the French official complains. "Intelligence meetings are more often than not bilateral, and you cannot say that there is a common European strategy against terrorism."
National intelligence-gathering agencies, on the other hand, are cooperating better with each other than in the past, and overcoming traditional rivalries. The British government has set up a Joint Terrorism Analysis Centre, billed as a "one-stop-shop" fed with intelligence from all Britain's secret services, and the French, German, and Spanish authorities have established similar organizations.
European judicial authorities, however, are having difficulty translating intelligence into evidence, as they begin to prosecute Islamist terrorist suspects. The German government was obliged to deport a Moroccan man back home last month when its case against him collapsed in court, and the largest-ever trial in Europe of suspected Islamist terrorists ended in Madrid this week after unearthing little hard evidence linking any of the 24 defendants to any crime.
• Dan Murphy contributed to this report from Cairo.