When European foreign ministers meet in a rare emergency session in Brussels this morning, they will face the unenviable task of making sense of yesterday's events in New York and Washington - and choosing the right response to them.
The ministers will have little difficulty in agreeing a statement condemning the attacks and expressing sympathy to the American people. But as the enormity of yesterday's outrage sinks in, many will share the judgment expressed by the External Affairs Commissioner, Mr Chris Patten, immediately after the attacks.
"This is one of those few days in life that one can actually say will change everything," he said.
For Europe, the most immediate consequences will be in the areas of security, economics and foreign policy.
European governments moved quickly yesterday to tighten security in their capital cities and potential targets, such as NATO's headquarters in Brussels and the Frankfurt stock exchange, were evacuated hastily.
EU governments have been considering how to improve co-operation between their police forces since this summer's violent protests in Gothenburg and Genoa.
But yesterday's attacks will provide a spur to intensifying those efforts and sharing more information about suspected terrorists.
Today's meeting could determine whether the EU's security response is decisive but measured or heavy-handed and potentially dangerous to civil liberties.
If the finger of suspicion continues to point to Islamic extremists, European governments will have to work out how to ensure that necessary security precautions do not lead to persecution and increased discrimination against Europe's Muslim population.
Within minutes of yesterday's first attack on the World Trade Center in New York, the international markets reacted dramatically.
Shares plunged, the price of oil and gold shot upwards and the dollar fell sharply against the euro.
The European Central Bank (ECB) declined to speculate on the potential economic consequences of the attacks but there is little doubt they could be almost as profound for Europe as for the US itself.
With the world economy teetering on the edge of a recession, yesterday's shock could be the event that tips the balance by choking off consumer confidence.
As economic growth has shrunk in the US and in Europe's biggest economies in recent months, it is the continued willingness of consumers to spend money that has prevented recession.
Yesterday's psychological blow is almost certain to affect US consumers, a development that will hit European experts. Consumers in Germany, Europe's biggest economy, are famously terrified of personal debt and it would take little to shake consumer confidence there.
A significant downturn in Germany could force the ECB to reconsider its rigid view that the sole function of its monetary policy is to control inflation, leading to a fall in interest rates.
Such a step could help to avert economic disaster but any change in the ECB's strategy would undermine further the institution's already shaky authority.
With weeks to go to the launch of euro notes and coins, any blow to the ECB's credibility would have an exaggerated impact on the reputation of the European institutions as a whole.
Meanwhile, if oil prices continue to rise, European governments can brace themselves for a repeat of last year's fuel protests - but this time in the context of a looming recession.
The most significant long-term impact of yesterday's events could prove to be on the EU's fledgling common foreign policy.
If, as many commentators have suggested, the perpetrators are Arab terrorists, the spotlight will turn on the EU's recently enhanced role in the Middle East peace process.
When President George W. Bush's government signaled its determination to step back from the process early this year, some EU officials welcomed the opportunity to boost Europe's role.
But the comprehensive nature of Washington's disengagement has taken Europe by surprise and left exposed the limits of the EU's influence on both sides.
Yesterday's attacks will intensify the urgency of the EU's peace- brokering efforts in the region but they could precipitate conflict with the US if Washington is tempted into an act of reckless vengeance that could dash any hopes of peace.
As they parted after their most recent summit in a lakeside hotel outside Brussels on Sunday, none of the ministers could have imagined they would meet just three days later in a world transformed.
And as they prepare to make decisions of great consequence today, most will be as bewildered as the rest of us about the real meaning of yesterday's horror.