US and British military planners are engaged in an unprecedented exercise to defeat an elusive enemy - the perpetrators of the ruthless terrorist attacks on New York and Washington and those states believed to have harboured them.
"We are positioning our assets to keep our options open," said a British defence source yesterday.
"We are not ruling anything in or out." That is not quite correct. One thing is clear: military action will not be limited to one-off strikes by cruise missiles such as those ordered by the Clinton administration after the attacks on US embassies in Kenya and Tanzania in August 1998.
Osama bin Laden remains the prime suspect behind those attacks. But the missile strikes on a pharmaceutical factory in Khartoum and southern Afghanistan are recognised, even by Washington, to have been a mistake and counterproductive.
It is also clear that this time Washington - and a key figure is Colin Powell, the US Secretary of State - will prepare the ground for military action by diplomacy.
Evidence from intelligence-gathering will also be needed if the US is to attract the broadest possible coalition - within the Arab world as well as NATO - for any military action.
"To use military force you have to be thoroughly convinced there is a state involved - it is not much use against a group," cautioned Rear Admiral Richard Cobbold, director of the UK's Royal United Services Institute.
"Don't expect anything over the weekend," a government source in London said yesterday.
But the pressures on President Bush to take some kind of military action soon are strong and growing. The British government is waiting for a decision, and requests, from Washington.
Defence sources said that, to begin with at any rate, Britain will provide "facilities, not assets".
Britain has already provided the US with the full resources of its security and intelligence agencies, including its listening post in Cyprus.
Military assets would include Diego Garcia, the British Indian Ocean territory which is a base for American B-52 bombers, the US base at Fairford in England, and the British air base at Akrotiri in south-west Cyprus.
Arab and Muslim countries, including Pakistan and even Saudi Arabia, might fight shy of offering their bases for US aircraft, as might Turkey, a NATO ally but also a Muslim country.
Pakistan is home to bin Laden's supporters, Turkey is concerned about Islamist opinion in the country, and even Saudi Arabia's western friends now concede that the presence of US bases there is provocative.
Russia, no friend of Islamist groups and of the Taliban in particular, would probably allow the use of bases north of Afghanistan.
But the US has floating bases in the region.
The aircraft carrier Carl Vinson is already in the Gulf, and the Enterprise, accompanied by cruise-missile submarines, will also be involved. Coincidentally, a British naval task force is steaming towards Oman for a long-planned exercise. The British carrier Illustrious is making for the Suez canal along with the cruise missile submarine Trafalgar, and the helicopter carrier Ocean with a commando brigade on board.
Military action could include bombing Afghanistan by manned aircraft, and the deployment of special forces troops.
Defence officials in Washington said strike options included a lengthy use of military forces on the land, at sea and in the air. They include the covert entry of special forces.
The US deputy defence secretary, Mr Paul Wolfowitz, said retaliation would continue until the roots of terrorism were destroyed.
"It's not just simply a matter of capturing people and holding them accountable, but removing the sanctuaries, removing the support systems, ending states who sponsor terrorism."
The US navy secretary, Gordon England, added: "This is not going to be a short programme."