When terrorists killed 191 people in Madrid in March 2004, the repercussions were rapid: Socialists stunningly defeated the pro-US government in that week's election, and soon withdrew Spanish troops from Iraq. There was little doubt in Spanish minds that the attacks were directly linked to the nation's involvement in the Iraq war.
Now, in the wake of the July 7 London bombings, the same question is being asked in Britain. Was the worst terrorist attack in London's history a direct consequence of robust British support for the US campaign in Iraq? Or were there deeper-seated causes?
Academics, Muslim leaders, and many Britons say Iraq was crucial.
A survey published in the Guardian newspaper Tuesday found that nearly two-thirds of respondents believed it was a factor. A report issued by the influential independent think tank Chatham House said there was "no doubt" that Iraq had made Britain's antiterror struggle more difficult.
Yet some say that approach is too simplistic. Al Qaeda-type terrorism - the 1998 US embassy bombings in Kenya and Tanzania, for example - long predates the Iraq war. And its targets have included Muslim countries such as Saudi Arabia. Some radical jihadis identify all non-Islamic states - not just Western ones - as enemies.
Some experts argue that Britain has been a target for years and that its policy toward countries such as Israel and Afghanistan may be as much to blame.
"The terrorism of the kind that led to the London bombings began long before the Iraq war," says Prof. George Joffe, an expert in the Middle East at the Center for International Studies at Cambridge University.
Nations that have sided with the US campaign in Iraq have found themselves in the firing line since 2003. Australia was targeted in an attack on its Jakarta embassy in September 2004; Spain suffered the Madrid bombings; British interests were attacked in Istanbul in November 2003. Coalition partners Denmark and Italy have been directly threatened in Islamist website postings.
Osama bin Laden has asked Western powers: "Why did we not attack Sweden?"
"[The Iraq war] gave a boost to the Al-Qaeda network's propaganda, recruitment and fundraising," asserts the Chatham House report, coauthored by terrorism experts Frank Gregory and Paul Wilkinson.
Many Britons agree. The Guardian survey, carried out by the ICM research group, found that 33 percent of Britons ascribed "a lot" of blame for 7/7 to the decision to invade Iraq, and 31 percent "a little." Only 28 percent said the Iraq gambit was not at all responsible for the London attacks.
There is a growing realization, moreover, that the Iraq war has not just ignited the fury of Al Qaeda members, but may have driven countless new recruits into their camp - like the four perpetrators of the 7/7 Tube and bus bombings.
Inayat Bunglawala of the mainstream Muslim Council of Britain lobby group says it has become much harder for moderates to persuade disaffected youth to maintain faith in the authorities when the authorities are seen to be responsible for Muslim deaths in Iraq.
"The Iraq war certainly helped Al Qaeda to recruit more people and boosted their propaganda efforts," he says. "It has undermined mainstream Muslim organizations."
The British government has dismissed suggestions that it brought terror upon itself by joining the Iraq campaign.
British Prime Minister Tony Blair prefers to speak of the enemy in terms of its "evil ideology," not its geopolitical ambitions. Some academics partly agree, noting that some Al Qaeda formulations seem more concerned with a religious-historic quest to spread the Islamic umma, or community, than with specific facts on the ground.
But Professor Joffe says that the overriding motivation for Islamist terrorism remains US and British policy in the Middle East.
Another academic, Robert Pape of the University of Chicago, supports this contention in a study of suicide bombings over the past 25 years. He found that 95 percent of all episodes were aimed at forcing occupying states to remove military forces from contested territory.
He noted that Iran, the world's foremost Islamic theocracy, has not produced a single suicide bomber. It is also not occupied by a Western force.
Some terrorism experts also partly support Blair's contention. If Iraq truly was the cause for the bombings, then antiwar countries like Germany and France should be able to rest easy. But they aren't. France's president, Jacques Chirac, warned recently that no country is safe from terrorism.
When asked if the London bombings might accelerate plans to withdraw from Iraq, Maj. Charles Heyman, a defense expert with Jane's Information Group in Britain, said: "You're joking. Surrender to a bunch of terrorists? Britain will not respond that way."
As Mr. Blair said this week: "We have got to be very careful that we don't enter into a situation where we think if we make some compromise on some aspect of foreign policy, these people are going to change.
"They are not going to change. They will just say 'They are on the run, let's step it up.' "