WASHINGTON - Two years after the United States launched a war in Iraq with a crushing display of power, a guerrilla conflict is grinding away at the resources of the U.S. military and casting uncertainty over the fitness of the all-volunteer force, according to senior military leaders, lawmakers and defense experts.

The unexpectedly heavy demands of sustained ground combat are depleting military manpower and gear faster than they can be fully replenished. Shortfalls in recruiting and backlogs in needed equipment are taking a toll, and growing numbers of units have been broken apart or taxed by repeated deployments, particularly in the Army National Guard and the Army Reserve.

"What keeps me awake at night is, what will this all-volunteer force look like in 2007?" Gen. Richard Cody, Army vice chief of staff, said at a Senate hearing last week.

The Iraq war has also led to a drop in the overall readiness of U.S. ground forces to handle threats at home and abroad, forcing the Pentagon to accept new risks - even as military planners prepare for a global anti-terrorism campaign that administration officials say could last for a generation.

Stretched by Iraq and Afghanistan, the United States lacks a sufficiently robust ability to put large numbers of "boots on the ground" in case of a major emergency elsewhere, such as the Korean Peninsula, in the view of some Republican and Democratic lawmakers and some military leaders.

They are skeptical of the Pentagon's ability to substitute air and naval power, and they believe strongly that what the country needs is a bigger Army. "The U.S. military will respond if there are vital threats, but will it respond with as many forces as it needs, with equipment that is in excellent condition? The answer is no,"said Sen. Jack Reed, a Democrat from Rhode Island.

To be sure, the military has also benefited from two years of war-zone rotations, and from a historical perspective it is holding up better than many analysts expected. U.S. troops are the most combat-hardened the nation has had for decades, and re-enlistment levels have generally remained high. The war has also spurred technological innovation while providing momentum for a reorganization of a military that in many ways is still designed for the Cold War.

Moreover, military leaders are taking steps to ease stress on the troops by temporarily boosting ranks; rebalancing forces to add badly needed infantry, military police and civil affairs troops; and employing civilians where possible. Friday, defense officials worried about recruiting announced that they will raise the age limit, from 34 to 40, for enlistment in the Army Guard and Reserve. The Pentagon is spending billions to repair and replace battle-worn equipment and buy extra armor, radios, weapons and other gear.

Yet such remedies take time, and no one, including senior officials, can predict how long the all-volunteer force can sustain this accelerated wartime pace. Recruiting troubles, especially, threaten the force at its core. But with a return to the draft widely viewed as economically and politically untenable, senior military leaders say the nation's security depends on drumming up broader public support for service.

"If we don't get this thing right, the risk is off the scale," said Lt. Gen. Roger Schultz, director of the Army National Guard, the military's most stressed branch.

A tough sell

At dusk the night the Iraq war started in March 2003, Staff Sgt. Spurgeon Shelley was near the Kuwaiti border, watching the orange glow of missiles streak overhead as he guided one Marine ammunition convoy after another north across the line of departure.

Manning a dirt berm while wearing his gas mask and full chemical suit, Shelley was determined to make it home alive to see his daughter, Lena, 2. "I'm going to do whatever I have to, to survive," he told himself.

Today, Shelley is on duty in what he calls a "one-man fighting hole" on another battlefield - a Marine recruiting station in Lexington Park, Md., in St. Mary's County - with a mission to persuade young men and women to enlist, and probably go to war.

One recent night, after making dozens of fruitless phone calls to high school students, Shelley said his recruiting job is more taxing than combat. "I hear 'no'more times in one day than a child would hear in their entire childhood," he said. "If I had hair, I'd pull it out."

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