US Defense Secretary Donald Rumsfeld said Feith advised him after the US presidential election in November that he planned to step down by this summer to return to private life.

"I'm hopeful he'll stay until we find an appropriate successor. We have not started looking for one," Rumsfeld told reporters.

A lawyer by profession, Feith has been an highly influential figure in the small circle of advisers who surrounded Rumsfeld as he led the United States into wars in Afghanistan and Iraq, while waging a broader campaign against Islamic extremists.

He left his mark on a range of controversial defense policies over the past four years -- the withdrawal from the 1972 Anti-Ballistic Missiletreaty, a new policy on nuclear forces, the policy of preemption to protect the United States against attacks with weapons of mass destruction, and the much criticized post-war planning for Iraq.

Most recently, he led planning for a sweeping overhaul in the way US forces are deployed overseas. The "global force posture review," as it is called, is expected to reduce the US military presence in western Europe, shifting it eastward to central and southeast Asia.

Feith has had his share of critics both outside the government and in.

Retired general Tommy Franks, who commanded the US-led invasions of Afghanistan and Iraq, memorably referred to Feith in a pep talk with military planners as "the dumbest (expletive) guy on the planet."

"Feith was a theorist whose ideas were often impractical; among some uniformed officers in the Building, he had a reputation for confusing abstract memoranda with results in the field," Franks wrote in his memoir, "American Soldier."

"Feith was a master of the off-the-wall question that rarely had relevance to operational problems," the general wrote, adding that he largely ignored his contributions.

Lawrence DiRita, the Pentagon's chief spokesman, said Frank's comments arose from the natural tensions between a commander and a civilian policymaker.

"No commander wants anybody in the Pentagon telling him what to do other than the secretary of defense," he said.

"Feith had a responsibility for policy oversight, he was instrumental in what he did, and I think if you're a commander focused on a war plan, your job is to sort of shield your planners from all the various policy guys that want to have a stake," he said.

"Tommy Franks would be the first to say how much he appreciated all the intellectual effort Feith put into all the things he did," he said.

DiRita said Feith enjoyed close relations with other military leaders but stood out for keeping the discussion on the war on terrorism above the purely military solutions.

"He has spent a lot of time inside the inter-agency process thinking about other ways to think about this battle of ideas," he said.

At a breakfast with reporters earlier in the day, Feith gave no hint he was preparing to leave.

He discussed the shifting priorities that will underpin a new strategy review that will shape the US military for years to come, saying it will give greater emphasis to terrorism and other unconventional threats rather than the usual conventional military threats.

He was asked how his thinking on preemption has changed in light of the failure to find weapons of mass destruction in Iraq.

"The issue of when the United States should act to deal with a problem is a really tough question, and I think gets really oversimplified in a lot of the discussion that focuses on the term preemption," he said.

"There are certain kinds of problems in the world that need to get addressed before they fully mature, and addressing them doesn't necessarily mean combat operations. It may mean diplomacy, it may mean different kinds of moves in the world, for example to interdict WMD transfers."

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