Washington is likely to rely mainly on its own, enormous military resources when it strikes back, despite Nato's unprecedented declaration of support for the United States.
However, the US has gone out of its way to create a coalition to fight international terrorism and it may want to bring other nations into its military response to provide "cover". The Canadian Foreign Minister, John Manley, said yesterday that his country would provide any troops requested.
But the multinational alliance is likely to be of more use in a longer-term strategy of coordinating intelligence and isolating states that harbour terrorists, than for masterminding a retaliatory strike.
Several factors enter the equation, both military and political. In practical terms the likely US targets, such as guerrilla training camps in Afghanistan, render most Nato allies of limited strategic importance.
A Nato source said yesterday: "It is hard to speculate but, in my view, they could do this all by themselves. Perhaps they might need something like overflying rights but they are perfectly capable of undertaking what they want by themselves."
During the Kosovo crisis, Nato states such as Italy were on the front line, providing vital military bases as well as equipment. This time only Turkey, a staunch US ally, enjoys a similarly strategic position.
The UK and Germany may be asked for intelligence help and, by coincidence, British military forces were heading for the Gulf for exercises. There will probably be more formal co-operation, including an agreement to allow Europol to tackle counter-terrorism and develop links with US agencies. But any military role for European allies is likely to be symbolic.
The Kosovo air campaign of 1999 illustrated the dominance of the United States within the 19-nation alliance, particularly in the air – the arena of most interest to the US at present. Indeed, so humiliated were European leaders by their impotence that they stepped up plans for an EU rapid reaction force of 60,000 men – although this will not exist until 2003.
Heavy-lifting equipment was almost exclusively provided by the US and even the contributions of Europe's two biggest military powers, the UK and France, were puny by comparison. Britain's lack of precision-guided armaments meant that less than 5 per cent of Nato sorties were British and only 2 per cent of the 1,000lb unguided bombs dropped by British forces hit their targets.
If the military contribution is small, the scope for political difficulties among the Nato allies is great. The alliance is an organisation of 19 nations, ranging from America's global superpower to an international minnow such as tiny Luxembourg. But it works by consensus and the objections of just one country are enough to block a Nato operation. For the US to use the "collective defence" clause invoked by the alliance on Wednesday would require two separate phases.
Firstly, America would have to prove they knew the identity of the culprit. Then Nato's North Atlantic Council would have to approve specific plans for a reprisal. Diplomatic pressure on countries such as Afghanistan, followed up by ultimatums, might be one thing; a bloody and indiscriminate strike another.
Already sentiment in Europe is urging caution. France, which prides itself in carving out a foreign policy distinct from the US, has already made it clear that it alone will decide whether French forces participate in any military activity.
France, like several other European countries, has a significant Muslim population – and presidential elections are looming next year. Even in the UK, which is likely to be a staunch ally of the US in almost any scenario, prominent voices are urging a proportionate response. Chris Patten, one of the UK's European Commissioner, has warned against the West plunging towards a "clash of civilisations". Jack Straw, the Foreign Secretary, (who represents a constituency with a big ethnic minority population), has pointed out that the US has no "blank cheque".
While the German Chancellor, Gerhard Schröder, expressed his strong support for America in the wake of the attacks on New York and Washington, he knows only too well the domestic sensitivity of supporting a military reprisal. German planes went into battle in Kosovo for the first time since 1945, a move which provoked intense debate.
Countries such as Spain, Portugal and Greece took a low-key role in Kosovo. Athens came under fierce pressure with polls showing 97 per cent of people to be against bombing.
With Kosovo, once Nato had decided on a course of action it stuck to it, remaining united despite numerous setbacks. But getting the initial agreement was not easy in 1999, and it cannot be taken for granted now.Reuse content