WASHINGTON — The war in Iraq is forcing top Pentagon planners to rethink several key assumptions about the use of military power and has called into question the vision set out nearly four years ago that the armed forces can win wars and keep the peace with small numbers of fast-moving, lightly armed troops.

As the Pentagon begins a comprehensive review that will map the future of America's armed forces, many Defense Department officials are acknowledging that an intractable Iraqi insurgency they didn't foresee has undermined the military strategy.

In the weeks after the Sept. 11 attacks, the Pentagon unveiled a new agenda that promised to prepare the military to fight smaller wars against terrorist networks and to swiftly defeat rogue states.

With Defense Secretary Donald H. Rumsfeld pushing for a "lighter, more lethal and highly mobile fighting force," the Pentagon scrapped as outdated the requirement that the U.S. military be large enough to simultaneously fight two large-scale wars against massed enemy armies. And it spent little time worrying about how to keep the peace after the shooting stopped.

Something happened on the way to the wars of the future: The Pentagon became bogged down in an old-fashioned, costly and drawn-out war of occupation. Though the rapid assault on Baghdad in March 2003 went smoothly, it is the bloody two years since that have diverged from the Pentagon's blueprint.

"When people were thinking about regime change, they really weren't thinking about the long-term stabilization and peacekeeping operations. There was a view that in terms of gross numbers, [regime change operations] wouldn't last as long as Iraq has," said Rand Corp. fellow Andrew Hoehn, who led the Pentagon's last major review in 2001.

As the Pentagon begins its assessment, it has 145,000 troops stationed in a country they were supposed to have left months ago. And with tensions rising between Washington and the two other countries labeled by President Bush as part of an "axis of evil" — Iran and North Korea — there is a growing belief within the military's ranks that the White House's rhetoric about preemptive war is out of sync with the U.S. military's strained resources.

Some inside the Pentagon criticized senior Bush administration officials for assuming that the war in Iraq would end when U.S. troops toppled Saddam Hussein's regime — and for assuming the U.S. could reduce its troop presence to 30,000 soldiers within six months of Baghdad's fall.

"The administration was flat wrong on Iraq because they had blinders on," said a senior Army official who worked on strategic planning at the Pentagon. "There's now a much greater perception that we need to know what we're signing up for before we get into it."

As a consequence, the importance of peacekeeping operations and help from allied militaries — ideas that some discounted three years ago as remnants of the President Clinton era — are back in vogue at the Pentagon.

Although born out of a blizzard of complex diagrams and flow charts, the Pentagon assessment, known as the Quadrennial Defense Review, or QDR, is not an academic exercise.

First undertaken after the collapse of the Soviet Union, the QDR is the playbook the Pentagon uses to guide decisions such as how big the military should be and which big-ticket weapons the Defense Department ought to purchase.

The Pentagon's decision in 2001 to scrap the two-war doctrine freed war planners from requiring enough heavy armor divisions to simultaneously fight two major wars, and allowed the Pentagon to invest in more futuristic weaponry like a missile defense system.

"We're always going to have a limited budget. So when we're making decisions about where to spend the next dollar, you want everyone clear about which sheet of music we're all singing off of," said Michele Flournoy of the Center for Strategic and International Studies. Flournoy was one of the lead Pentagon officials on the 1997 review, which embraced the two-war doctrine.

The new review, which is just beginning, will not be completed until early next year. Last fall, a Pentagon advisory board predicted that the protracted stability operations underway in Iraq and Afghanistan were a model for the U.S. military's future. The Pentagon has focused too little on preparing for what happens after major combat operations end, said the Defense Science Board, which advises Rumsfeld.

"Some have believed, or hoped, that the technological and conceptual advances … can reduce the time and personnel needed for stabilization and reconstruction," the board said. "Unfortunately, we do not find that is the case."

The Defense Science Board report was commissioned to guide the upcoming Quadrennial Defense Review studies, and it is part of a growing body of Pentagon analysis signaling a shift in Defense Department thinking.

Another possible shift has to do with the perception of U.S. allies. With the Army and Marine Corps straining to meet the Pentagon's troop requirements for Iraq and Afghanistan, the participation of allies has taken on greater importance. Foreign troops would be necessary for any large-scale operation the U.S. military might undertake, planners said, if only to share the post-conflict burdens such as those confronting the U.S. military in Iraq.

"There are smarter, more efficient ways to do regime change and occupation," said one senior civilian official at the Pentagon. "One of those ways is to rely much more on our friends and allies to do the back-end work."

In recent weeks, Bush administration officials have taken a far more conciliatory tone with some of America's oldest European allies. Whereas Rumsfeld once slighted NATO's western European members — referring to them as "old Europe" — he poked fun at those comments to win over European ministers during a trip to the continent last month.

"That was old Rumsfeld," he said.

On Thursday, Rumsfeld welcomed French Defense Minister Michele Alliot-Marie to the Pentagon, praising the cooperation between the nations' militaries over the years.

The Iraq war has also shown the weakness in a strategy created by the Pentagon in 2003 to help plan major operations.

The 10-30-30 construct said that the U.S. military should plan military actions to seize the initiative within 10 days of the start of an offensive, achieve limited military objectives within 30 days, and be prepared within another 30 days to shift military resources to another area of the world.

Many Pentagon officials fear that the success Iraqi insurgents have had in preventing a U.S. troop reduction in Iraq could be the new rule, rather than the exception.

As few enemies choose to fight the U.S. military head-on, they might opt instead to fight protracted rear-guard insurgencies.

"I think that the Pentagon realizes by now that 10-30-30 is largely outdated," said Frank Hoffman of the Marine Corps' Center for Emerging Threats and Opportunities, a contributor to the Defense Science Board study. "It presumes a model of warfare that we ourselves have made obsolete."

Hoffman said no adversary was likely to present U.S. forces with a conventional threat that can be defeated in 30 days.

"Our enemy's metric is protracting conflicts to 3,000 days or more," he said. "Prolonged insurgency, death by a thousand cuts, is their answer to 'shock and awe.' "