For the US military, yesterday was a Hollywood horror movie played in real time; everything Washington feared and planned for during the past decade has happened, in a matter of a few hours. And for the ordinary American public, yesterday's events will rank equal to the attack on Pearl Harbor. The world's only remaining superpower, the country whose territory appeared invulnerable, has been grievously wounded.
As all events of such epic proportions, yesterday's tragedy had its own bitter ironies. For years, the US pushed governments around the world to tighten airport security. Indeed, US federal law obliges American airline companies to formally warn passengers of the dangers inherent in travel to certain foreign airports which Washington deems insecure.
But the security that is demanded of others never applied to US domestic flights. And lobbying from American airlines, who feared loss of business, apparently prevented the adoption of new safety measures which the Clinton administration suggested only a few years ago.
The US also runs a vast intelligence operation; more than 100,000 officials intercept, translate, decode and analyse every scrap of human and electronic information obtained by satellites or spies and collated around the world and around the clock. For at least a decade, the chief aim of this operation was to identify international terrorist networks.
There is some evidence that Washington knew for a few days that something was afoot; its embassies in Asia and Europe were warned of an impending attack, and some preparations were made in the US as well. But it is now patently obvious that the Americans failed to penetrate the terrorist organisation that was responsible for the atrocities.
Of course, penetrating such organisations is the most difficult job of any intelligence agency. But yesterday's attacks were not ordinary; they represented the most stupendous action in the two-century history of terrorism. And they must have involved an extensive network of activists who not only planned the operation with great care, but were also prepared to die while executing it.
More importantly, the network included highly sophisticated people, some able to fly aircraft, some able to navigate, and some experts on construction, for they made sure that the World Trade Center towers were hit at their most vulnerable points. Either way, the operation was planned for months if not years, and all the US intelligence agencies failed to detect it.
In the weeks to come, and in the time-honoured American way, committees of inquiry will pore over every detail, every scrap of information. Tomes of learned reports will be published, and some officials will either hand in their resignations or be fired.
Yet, the impact on US military planning, and America's view of the world, will be much more significant and long-lasting.
And here, again, one is struck by more ironies. Ever since it came to power, the Bush administration has advocated the implementation of a vast missile defence programme. Its chief argument was that America's continental mass will increasingly be threatened by missiles developed by so-called "rogue states" - countries with no international responsibility, but an enduring hatred of the US. Opponents of the missile defence programme often retorted that the real threat to the US will not come from the odd rickety North Korean or Iraqi missile, but from acts of terrorism.
The US public was divided, and the debate over missile defence was scheduled to start in Congress at the end of this month. Yesterday's atrocities have rendered the entire argument redundant; the missile plans have been buried in the rubble of the Pentagon. On the one hand, President Bush will get whatever money he wants for new defence programmes. But public pressure will also ensure that the first priority will be on beefing up America's internal security and fighting terrorism.
US military planners already have an extensive programme entitled "Homeland Defence". This will now receive much more cash, probably more than it can readily spend. Suddenly, Americans feel vulnerable. And, as so often in the past, their first instinct will be to throw money at the problem.
This reaction will also have foreign policy consequences. For at least the next few months, it will be impossible to get the US to focus on any major international security issue that does not involve the fight against terrorism in general, and the hunt for the culprits of the massacre in particular.
The idea that a US president would discuss the force that needs to stay in Macedonia after the end of this month is ludicrous. The Balkans, the Baltics and Russia, the key issues that preoccupy Europeans, are, at least for the moment, off the agenda. Pearl Harbour persuaded America to discard its isolationism and enter the second World War; yesterday's attacks, although likely to have just as profound an impact on American minds, will push the country towards greater isolationism. Sorting out bandits in Macedonia may be fine, but protecting US citizens at home comes first.
The Europeans can play a role in the next few months, provided that they understand these American reactions and try to guide them along a constructive path. The US administration is certain to demand quick international co-operation against terrorism, and Europe should be swift to reciprocate.
The prospects for the emergence of a truly effective global anti-terrorist regime are, however, slim; as always, one country's terrorist is another country's "freedom fighter". Previous anti-terrorist discussions between Russia and the US foundered over precisely what constituted a terrorist: for Moscow, the only terrorists worth eliminating were the Chechens, while the US had other candidates in mind. The chances are that any new negotiations on the subject will reach a similar impasse.
Nevertheless, European support for whatever diplomatic effort Washington launches in the near future will be essential, in order to prevent a drastic American move against any country that may be identified by the US as being even remotely responsible for harbouring those who have perpetrated yesterday's murders.
Staying the course, persuading ordinary Americans that their grief is Europe's grief and trying to refute the idea that the US now stands alone against a world populated by madmen who are often Muslims should now be the key priorities. Over the long term, however, European governments should try to influence the debate over America's military posture. Terrorism is bred by long-standing grievances; these need to be addressed by increasing US economic aid to key international hotspots, rather than just beefing up America's defences, or tearing up international disarmament agreements as Washington originally proposed.
Yet America's fundamental security dilemma is only beginning. For decades, the US had to reckon with opposition from other countries, and with the possibility of annihilation. Today, US power is so overwhelming that terrorist activities offer the only hope for the world's discontented to hit the country. The sight of the world's only superpower, both reigning supreme and terrorised by a handful of madmen at the same time, is the ultimate irony of the post-Cold War world.
Jonathan Eyal is director of studies at the Royal United Services Institute in London