The Madrid train bombings of March 11, 2004 were Spain's worst acts of terrorism in modern times. Spain's then-ambassador to the United States, Xavier Rupirez, likened the attacks to what happened in the U.S. on September 11, 2001. "This is our 9-11 because of the people involved, because of the number of killings, the willingness to kill people going to work," he remarked. 191 Madrid train commuters were killed and more than 1800 others injured. Spain, and in a broader sense Europe would never be the same.

The bombings happened on a Thursday morning three days before Spain was to hold national elections. Prior to the attacks, the conservative government of Prime Minister Jose Maria Aznar was expected to retain power, if narrowly, against the Socialists. Right after the blasts, Prime Minister Aznar put the blame on the Basque separatist group ETA, which had been linked for years to acts of terror. ETA denied any role in the bombings, and two days later a videotape found near a Madrid mosque disclosed that al-Qaida had attacked the trains to punish Spain for its involvement in Iraq and Afghanistan.

Simon Serfaty at the Center for Strategic and International Studies in Washington says Prime Minister Aznar's accusation against ETA doomed him at the polls. "There was a ten percentage point switch during the 72 hours between the blasts and the election which was a rebellion against the perception that this is what Aznar was attempting to do. That brought him down," he says.

Spanish voters swept in the Socialist Party, whose leader Jose Luis Rodriguez Zapatero became the next Prime Minister. He quickly announced he would withdraw Spain's 1300 troops from Iraq, a deployment many Spaniards had opposed. Esther Brimmer at Johns Hopkins School of Advanced International Studies in Washington says Spain's decision influenced other countries with forces in Iraq.

"When it became clear that Spain was going to pull out of Iraq, what we've seen in the year after that is it became more acceptable for other countries to go home. It then created greater pressure on the other Iraq contributors who are still there," she says. 

Across the European Union, the Madrid bombings brought the threat of mass terror to the forefront. Charles Cupchen at the Council on Foreign Relations in Washington says the E.U. and the United States found common ground. "The Europeans heightened concerns about terrorism, saying 'we are not immune to what hit the United States,' he says, adding "In that sense, pushing the Europeans closer to the U.S."

Mr. Cupchen says the Madrid bombings also brought the states within the European Union closer together. "They've ramped up efforts to share intelligence and to get more police cooperation throughout the E.U. That has moved forward," he says.

Investigators announced soon after the Madrid bombings that a group of Moroccan Islamists were responsible. For Europeans already uneasy about the growing presence of Muslims in their midst, this was unwelcome news. Then adding to concerns, a controversial Dutch film director, Theo Van Goh, was killed last November after making a movie critical of Islam. A Dutch Moroccan was arrested for the murder.

There are at least 20 million Muslims already in Europe, and millions more are expected to arrive. Esther Brimmer at Johns Hopkins' School of Advanced International Studies says tensions are clearly rising.  "We're seeing both increasing anti-Muslim sentiment and Muslim extremist violence in the name of Islam," she says. "This has created deep social tensions. I think that's a real long-term concern."

After the Madrid bombings, France passed a law preventing Muslim girls from wearing headscarves in school. Charles Cupchen at the Council on Foreign Relations says the full integration of Muslim Europeans is in doubt. "The Europeans don't think about multi-ethnicity the same way the Americans, for example, do," he says, adding "They're not prepared to have a multi-cultural society. They're not comfortable with that yet."

The Madrid bombings have caused second thoughts about Muslim Turkey joining the E.U. Europeans have been worried that Turkish membership might lead to a flood of Muslim immigrants. Now there are profounder fears that Islam and Europe may be basically incompatible. Such are the consequences of terror.