With US forces mobilising around the world, the economy nosediving, and a broad but potentially fragile international coalition assembled against terrorism, he aimed to strike the delicate balance of calming the fears of a country still in shock and steeling Americans for a difficult and costly war ahead.

After a meeting with congressional leaders on Tuesday, he pledged to use the speech to "urge our fellow Americans to go back to work and work hard", while emphasising the need to be on alert while the government "runs down every lead, every opportunity to find someone who would want to hurt any American".

Both Mr Bush's approval ratings and public support for military retaliation for last week's attacks are high and the president is keen to forestall any criticism of inaction. But he is emphasising that the conflict the US military is about to embark upon is unlike any in its history. With the military strategy as yet unclear and undeclared, and an enemy which is either elusive or evasive, it is vital for the president to retain the confidence of the public.

"The mindset of war must change," Mr Bush said on Wednesday. "It is a different type of battlefield. It is a different type of war." The battles, he said, "will be fought visibly sometimes, and sometimes we'll never see what may be taking place".

His national security adviser, Condoleezza Rice, said his speech would seek both to reassure the public and dispel expectations of a swift victory.

"This is an opportunity to talk to the American people about the kind of threat that we face," she said. "An opportunity to urge patience and reason to demonstrate again that his resolve is going to be over a long period of time."

The speech would "make the case for why we are entertaining this long struggle", said Ms Rice, who is anxious to discourage simplistic comparisons with past conflicts.

"This is not Pearl Harbor", she said, discouraging comparisons with Franklin Roosevelt's "date which will live in infamy" speech to Congress, which declared war on Japan. "There are no beaches to storm and islands to take ... [This will be] a war of will and mind," she said.

Last week a previously bitterly divided Congress united to approve the use of military force in response to the terrorist attacks, and set aside $40bn for rebuilding and retaliation. The Senate passed the resolution with no dissent or debate, while the House of Representatives endorsed the measures by 470 votes to one.

This bipartisan unity has remained, although cracks are beginning to appear. There are concerns that the administration's anti-terrorism plans could impose unreasonable infringements on civil liberties. The Democratic chairman of the Senate judiciary committee, Patrick Leahy, voiced his opposition to the proposals to detain and deport immigrants suspected of terrorism. "We do not want the terrorists to win by having basic protections taken from us," Mr Leahy said.

A growing alliance of legislators and pressure groups from both the left and the right have questioned whether the government is moving too fast in curtailing civil rights to combat terrorism.

Division has also surfaced over the administration's proposals to stimulate the economy by cutting capital gains tax, which is opposed by Democrats. The economic dimension, will become particularly important as the recession begins to bite. Plunging share prices and layoffs of thousands of airline workers presage a sharp slowdown after several years of unprecedented growth.