It will be a war like none before it, global in scope, costing billions of dollars - and one in which neutrality will be difficult. Yet the enemy in military terms is tiny: a shadowy band, whose main armament is suicidal fanaticism.

There can be little doubt that George W. Bush was in deadly earnest when he declared war on terrorism this week, even if there was also an element of rhetoric in the president's description of the struggle ahead. He has persuaded Congress to vote him an extra $40bn. He has invoked Article 5 of the North Atlantic treaty, which requires the US's allies to help him. He has called up 50,000 reservists. He has placed two airborne divisions on alert. And he is pouring oil stocks into Diego Garcia, the US base in the Indian Ocean.

Yet the rules - and wider consequences - of the war that may follow Tuesday's attacks on New York and Washington remain unclear. The domestic implications are the easiest to see. The president was seeking language that would catch the mood of outrage expressed by the people of America - and the world.

But he was doing more than that. He was warning implicitly that the military response could involve the sacrifice of American servicemen's lives. This would be an abrupt change from the popular presumption that US military might could be deployed with minimal casualties, as it was in the Gulf war, in Kosovo and in missile launches in 1998 against terrorist camps in Afghanistan and Sudan.

This time, the administration says, the campaign will be "broad and sustained". As Paul Wolfo-witz, the deputy defence secretary, put it: "You don't do it with just a single military strike, no matter how dramatic." If the US is serious about destroying terrorist bases, it may have to deploy ground troops. And the mood of the country appears to be responding to this need. Some 70 per cent in a recent poll agreed that US casualties might have to be accepted.

Curtailment of liberty

The other domestic implication of Mr Bush's declaration of war is that curtailments of ordinary liberty may have to be imposed. These could range from identity cards to a renewed attempt to give the authorities the key to all encrypted messages on the internet. Like increased airport security, some of these measures may be a necessary inconvenience - but there are dangers in going too far. In a modern state individual freedoms are eroded by small degrees and are difficult to recover.

The international implications are more imponderable and may be profound. By declaring war, Mr Bush is suggesting that normal international rules - for example on the extradition of suspected criminals - might be suspended. War sweeps aside such niceties, nor does it respect the sovereignty of those countries deemed to be friends of the enemy.

Formal enemies

This poses a threat, not only to the Taliban in Afghanistan but also to several other regimes that the US suspects of giving help or encouragement to terrorists. By using the language of war - and the worldwide sympathy for its cause - the US will force such countries to make a choice. Those who are not allies may find themselves formal enemies of the most powerful military machine the world has known.

Colin Powell, US secretary of state, said this week that he would approach the president of Pakistan "as if I am talking to a friend and let a friend know what we would like to see happen". It is an offer that may be difficult to refuse.

But to maintain such pressure and to achieve success against an elusive enemy of presumed Islamic militants, the US needs two things. The first is the continued support of the larger powers, including Russia, that has been reflected in a UN resolution as well as in Nato and the European Union.

The second is to try to keep moderate Arab opinion on its side. Any attempt to revive Middle East peace talks, however forlorn, could help in this.

More important, the US must proceed with careful planning and the broadest consultation among its allies before any military engagement. If it shoots from the hip, as the Clinton administration did, it will kill few terrorists and risk losing support even among its allies.

To sustain the legitimacy of this new kind of war, Mr Bush will need to be very clear about its detailed objectives and how they are to be achieved - by joint action wherever possible.

The US may be an unassailable military power but in this fight warships and missiles will not be enough. It must also defend the moral high ground. And that will require a carefully focused response in which concern for civilian lives is set alongside the search for justice at the heart of America's purpose.