WASHINGTON (Reuters) - Before he has turned 30, U.S. Army Capt. Jaron Wharton has served two yearlong tours in the Iraq war, felt the severe strain on soldiers and seen the Army change the way it fights.

A US soldier searches a house during a patrol in Baghdad November 10, 2007. REUTERS/Stefano Rellandini

In many ways, Wharton’s experiences mirror those of the Army as a whole in the five years since U.S. troops invaded Iraq and raced to Baghdad to topple Saddam Hussein’s regime.

“Our patrols were treated like floats in a Macy’s Thanksgiving Day parade,” Wharton recalled of the early days. “People were really, really thankful.”

But as the scenes of jubilation gave way to a deadly insurgency, Iraq has become the biggest test the U.S. military has faced without a draft since the fight for independence from Britain in the 1775-83 Revolutionary War.

Nearly 4,000 U.S. troops have died in Iraq, more than 3,200 of them killed in action. Almost 30,000 have been wounded.

When he returned to Iraq for his second tour in November 2005, Wharton found a tougher environment. One sergeant was hit by three bomb attacks during an 18-hour patrol, he recalled.

Some U.S. soldiers are now on their third or fourth tours. Deployments have been extended from a year to 15 months, with only a year at home between tours.

The long deployments take a heavy toll on personal relationships, said Wharton, whose wife is also an Army captain who has served two tours.

“In most of these deployments, the girlfriend, she’s not there at the end,” said Wharton, 29, of Hoover, Alabama, who has also served in Afghanistan. “Fiancees are hit or miss. And sometimes even wives are gone.”

More than one in four soldiers on their third or fourth Iraq tours suffer mental health problems, an Army survey found.

MAJOR IMPACT

As well as the personal toll on soldiers, the war has had a major impact on the military as an institution, above all on the Army, which has done most of the fighting.

It exposed a medical system that was ill-equipped to provide long-term care for a large number of wounded warriors.

It has eroded the U.S. military’s ability to fight another major war. Commanders have said the Air Force and Navy would likely have to take the lead if a new conflict arose, because ground forces are so stretched.

The strain has prompted the United States to start expanding the Army by 65,000 active-duty troops for a total force of 547,000. The Marine Corps is also growing.

Despite the war, the Army has continued to meet recruitment goals, although critics say it has lowered standards to do so.

It has offered cash incentives to attract recruits and hold on to mid-level officers like Wharton, who is now on a fellowship at a Washington think tank, the Center for a New American Security, but remains a member of the Army.

“Folks are amazed by the resiliency of our soldiers,” he said, sitting in a Pentagon cafeteria. “You’ve got to commend these soldiers because they’re strong and they keep at it.”

FUNDAMENTAL DEBATE

Beyond the strains of the war, Iraq has also sparked a fundamental debate about the future of the Army.

“We have an Army that was designed to kill people and break things and we still need an Army that can do that,” said Lt. Col. John Nagl, the author of a book on fighting insurgencies called “Learning to Eat Soup With a Knife.”

“But we also, increasingly, need an Army that can help people and build things.”

That view has increasingly shaped U.S. strategy in Iraq, particularly since another counterinsurgency expert, Gen. David Petraeus, took command in Baghdad early last year.

Backed by a surge of 30,000 extra U.S. troops, Petraeus moved his forces off big bases to protect Iraqis and help provide essential services.

The extra troops and new approach have contributed to a steep decline in violence in Iraq since last summer.

Spurred by Iraq, senior officers have embraced the idea the military must be ready to do more “nation-building” -- once a taboo in the Bush administration -- to stabilize fragile states that could become a haven for Islamist militants.

“The greatest threat to international security has been states that are too strong,” Nagl said. “In the 21st century, I would argue that at least as great a threat, conceivably even a greater threat, is states that are too weak.”

Petraeus oversaw a new counterinsurgency manual, co-written by Nagl, that stressed the importance of noncombat missions and was issued in December 2006. The Army released a broader manual last month that also upgraded stability operations.

Some officers and analysts say the Army still needs much more reform to ditch Cold War thinking and embrace unconventional warfare. Nagl wants a 20,000-member corps created to train the militaries of friendly nations.

“Although we have an Army now that understands that it needs to be able to build and protect, we have not yet created the Army that can do those things,” Nagl said.