Senior American intelligence and defence officials believe that President George W Bush and his inner circle are taking steps to place America on the path to war with Iran, The Sunday Telegraph has learnt.
Pentagon planners have developed a list of up to 2,000 bombing targets in Iran, amid growing fears among serving officers that diplomatic efforts to slow Iran's nuclear weapons programme are doomed to fail.
Pentagon and CIA officers say they believe that the White House has begun a carefully calibrated programme of escalation that could lead to a military showdown with Iran.
Now it has emerged that Condoleezza Rice, the secretary of state, who has been pushing for a diplomatic solution, is prepared to settle her differences with Vice-President Dick Cheney and sanction military action.
In a chilling scenario of how war might come, a senior intelligence officer warned that public denunciation of Iranian meddling in Iraq - arming and training militants - would lead to cross border raids on Iranian training camps and bomb factories.
A prime target would be the Fajr base run by the Iranian Revolutionary Guard Quds Force in southern Iran, where Western intelligence agencies say armour-piercing projectiles used against British and US troops are manufactured.
Under the theory - which is gaining credence in Washington security circles - US action would provoke a major Iranian response, perhaps in the form of moves to cut off Gulf oil supplies, providing a trigger for air strikes against Iran's nuclear facilities and even its armed forces.
Senior officials believe Mr Bush's inner circle has decided he does not want to leave office without first ensuring that Iran is not capable of developing a nuclear weapon.
The intelligence source said: "No one outside that tight circle knows what is going to happen." But he said that within the CIA "many if not most officials believe that diplomacy is failing" and that "top Pentagon brass believes the same".
He said: "A strike will probably follow a gradual escalation. Over the next few weeks and months the US will build tensions and evidence around Iranian activities in Iraq."
Previously, accusations that Mr Bush was set on war with Iran have come almost entirely from his critics.
Many senior operatives within the CIA are highly critical of Mr Bush's handling of the Iraq war, though they themselves are considered ineffective and unreliable by hardliners close to Mr Cheney.
The vice president is said to advocate the use of bunker-busting tactical nuclear weapons against Iran's nuclear sites. His allies dispute this, but Mr Cheney is understood to be lobbying for air strikes if sites can be identified where Revolutionary Guard units are training Shia militias.
Recent developments over Iraq appear to fit with the pattern of escalation predicted by Pentagon officials.
Gen David Petraeus, Mr Bush's senior Iraq commander, denounced the Iranian "proxy war" in Iraq last week as he built support in Washington for the US military surge in Baghdad.
The US also announced the creation of a new base near the Iraqi border town of Badra, the first of what could be several locations to tackle the smuggling of weapons from Iran.
A State Department source familiar with White House discussions said that Miss Rice, under pressure from senior counter-proliferation officials to acknowledge that military action may be necessary, is now working with Mr Cheney to find a way to reconcile their positions and present a united front to the President.
The source said: "When you go down there and see the body language, you can see that Cheney is still The Man. Condi pushed for diplomacy but she is no dove. If it becomes necessary she will be on board.
"Both of them are very close to the president, and where they differ they are working together to find a way to present a position they can both live with."The official contrasted the efforts of the secretary of state to work with the vice-president with the "open warfare between Colin Powell and Donald Rumsfeld before the Iraq war".
Miss Rice's bottom line is that if the administration is to go to war again it must build the case over a period of months and win sufficient support on Capitol Hill.
The Sunday Telegraph has been told that Mr Bush has privately promised her that he would consult "meaningfully" with Congressional leaders of both parties before any military action against Iran on the understanding that Miss Rice would resign if this did not happen.
The intelligence officer said that the US military has "two major contingency plans" for air strikes on Iran.
"One is to bomb only the nuclear facilities. The second option is for a much bigger strike that would - over two or three days - hit all of the significant military sites as well. This plan involves more than 2,000 targets."