Newspapers in Spain and other European countries are describing the devastating bomb attacks in Madrid as Europe's 11 September - or 11-M, standing for 11 March.
America's war on terror has been seen differently on this side of the Atlantic. So how far will that change now?
Many Europeans share Spain's shock
The suicide hijackings directed at New York and Washington came as a huge shock to Americans. Before 2001, their sense of invulnerability at home was virtually intact.
In contrast, Europeans have lived with the threat of terrorism for many years - from left-wing extremists in Germany in the 1970s, from neo-fascist militants in Italy, from the IRA in Northern Ireland and mainland Britain.
European governments have taken extra security measures in response to America's 9/11 and their peoples have become accustomed to that.
The Madrid bombings are not on the scale of 9/11. But they are marked by the same ruthless savagery and intent to inflict maximum harm.
That is undoubtedly a shock to people in Europe - even if the attacks were the work of Eta, it is a new Eta.
Governments across Europe are now rethinking their strategies
As the German newspaper Die Welt put it, what happened was the "al-Qaeda-isation" of European terrorism.
Public opinion may therefore be readier to accept more draconian security and restrictions on civil liberties, though that cannot be taken for granted.
There has been much criticism of Britain's decision in anti-terrorism legislation to opt out of a section of the European Convention on Human Rights - the only country to do so.
Governments across Europe are now rethinking their strategies.
For example, a huge operation was already under way with international involvement to protect the Olympic Games in Athens in August.
Now the Greek government says the plan will be strengthened. It has asked Nato to help with security, for example in aerial surveillance.
The French government is calling in the military to reinforce police security for public transport.
The Italians have told the police and local authorities to tighten their precautions.
On a European level, some will make the case for more intense co-operation against suspected terrorists through the police agency Europol, and other EU institutions, as a matter of routine.
Public reaction to the Madrid bombings would be more significant if it turned out that they were actually carried out by Islamic militants.
"That would be a new situation," in the words of the German Interior Minister, Otto Schily.
The same would apply if it were shown that Islamic militants were working with Eta - with a home-grown European group.
Most vulnerable to criticism would be the governments of Britain, Spain and Italy, which strongly supported the US and the invasion of Iraq.
UK Prime Minister Tony Blair would point to a speech he made last week, emphasising the continuing threat of global terrorism and describing it as "a new type of war".
But the Spanish government would have to deal with the accusation that its policies had made the country an al-Qaeda target.
Rallying public support against Eta, the familiar enemy, is a more comfortable situation to be in.