"We know that the guys we train are some of the same dudes who are putting bullets in the back of people's heads or going to work on them with power drills," he wrote in a July 2006 e-mail home.

As the months passed, his cynicism and anger grew. "This place is now rotten to the core," he concluded.

A year later Cooper was back in Iraq, working with 150-man police unit. His second tour, which coincided with a surge of about 30,000 U.S. soldiers, almost felt like a different war. Violence dropped. Markets opened. Something resembling stability seemed to take hold. Cooper remembers his battle-scarred Iraqi police partner from that period as a brother in arms. The 33-year-old Green Beret is part of a generation of Army officers whose careers have been defined by the chaos and contradictions of one of America's longest and costliest wars. These soldiers have watched close friends die, weathered separations from family and struggled to explain to grieving parents why their loved ones didn't make it home.

As the United States marks the official end of combat operations in Iraq on Tuesday, how these officers and others like them make sense of the last seven years of war will profoundly affect how America wields military power over the next quarter-century.

So far, the lessons they are taking from their Iraq experience are ones of humility and caution that surpass the post-Vietnam War period. In Vietnam, a generation of officers blamed their defeat on a fickle American public and civilian leaders who placed limits on their use of firepower. Today's Iraq veterans are more apt to acknowledge that the limitations are their own.

They are slow to call Iraq a victory. "We had military successes, but the Iraqis will decide whether it is a long-term success or not," said Maj. Joseph Da Silva, who logged three tours.

And they are intimately aware of the human costs. "To realize how random and crazy all this is sparks that hint of panic," Capt. Casey Thoreen wrote in 2009 after a roadside bomb killed two of his men.

Among them, Cooper, Da Silva and Thoreen have spent nine years deployed -- seven in Iraq and two in Afghanistan. They have worked as staff officers and commanded in some of the war zone's most remote and austere outposts.

"If Iraq is to teach us anything, it must be that a new idea cannot be beat into a society," Cooper wrote in an e-mail in 2006. Today the onetime Rhodes scholar is finishing a doctorate at Harvard and teaching at West Point. For him and others of his generation, it is the dark days of the war, more than the successes of 2007, that dominate their memories of Iraq.

CAPT. CASEY THOREEN

Thoreen, who was finishing a training course when the March 2003 invasion began, joined his unit in early summer. Seven hours after he landed at an airfield near the northern Iraqi city of Kirkuk, a Humvee ferried him to a safe house where he met up with the 30-soldier platoon he was going to lead.

His platoon fell into a familiar rhythm for the U.S. occupation. At night, they raided houses, searching for insurgents. By day they patrolled and oversaw the construction of a school and a health clinic. Attacks on Thoreen's troops gradually increased.

In December 2003, U.S. commandos captured a haggard and defeated-looking Saddam Hussein outside of Tikrit. Jubilant U.S. commanders heralded the arrest as a major blow to the budding insurgency. The day after Saddam was detained, Thoreen returned from a patrol to learn that his first sergeant had fired Thoreen's Iraqi interpreter, a 36-year-old Kirkuk native who had been his guide to the city. Her sin: Some soldiers said they heard her expressing dismay at the way an unkempt Saddam had been shown on television by his U.S. captors.

Thoreen was furious. He knew his interpreter, like many nationalistic Iraqis, had mixed feelings about the U.S. occupation. But she wasn't a backer of the Iraqi dictator and had been happy to see him toppled.

Thoreen, the son of a Seattle lawyer, had grown up in a politically liberal household before attending West Point. He brought a healthy skepticism to his mission, one of his former brigade commanders said. In Iraq he worried that his fellow soldiers had begun to treat the Iraqis they had come to liberate with disdain.

The two sides seemed perpetually separated by a razor-wire divide. "We had no relationship with the Iraqis," said Thoreen. "If we couldn't interact with them as humans, we were never going to be successful."

The gulf felt especially wide after a botched raid. "I can only imagine the terror we inflict on innocent families when suddenly they have a squad of soldiers breaking through their door with weapons at the ready," he wrote in a December e-mail to his parents. To Thoreen's unceasing surprise, the Iraqis often asked the uninvited Americans to sit for tea. The young officer always declined.

"I feel so bad about disrupting their lives that I try to leave as soon as possible," he wrote.

MAJ. JOSEPH DA SILVA

In fall 2005, the American prospects for stabilizing Iraq appeared bleak. The Iraqi army was a mess, insurgent attacks were rising and the initial hope that the newly elected Iraqi government might undermine support for the insurgency had been exposed as wishful thinking.

Then-Capt. Da Silva, who had led a platoon of 16 soldiers in the invasion, was heading back to Iraq for his second tour. Da Silva, the son of a high school janitor, had been the president of his class at West Point. He had broad shoulders and close-cropped black hair and spoke with a hint of a New England accent.

He approached his job with an earnestness that sometimes bordered on naivete. "Every day I got to work with some of the brightest, most intelligent and hard working people I know ... men and women who do not know how to fail," he wrote a few days before he left for Iraq in September 2005. "They will make it work."

Within months, his faith in his Army had been crushed. As a battalion staff officer, Da Silva was stuck at headquarters monitoring the radios and calling in medical and attack helicopter support for troops under fire. The carnage was so constant that he began to plan his bathroom breaks around those moments when all of the battalion's squads were on base or in meetings with Iraqis, where they were less likely to be attacked.

His low point came when troops from his unit found a roadside bomb, and Da Silva called in engineers to destroy it. Before they could do their job, one of Da Silva's superiors grabbed the radio and ordered the soldiers to pack the bomb with extra explosives, Da Silva recalled.

The officer wanted to create a blast that the Iraqis in the area would hear and feel. The explosion leveled the wall of a nearby school that had been built by another U.S. unit. "I had returned to Iraq thinking I would find massive improvement, but what I saw was a state in anarchy and, even more disheartening, an Army that refused to learn," Da Silva wrote in an essay several years later.

Da Silva's battalion commander noted the increased number of arms caches that the battalion was finding and rising insurgent arrests as signs of progress. Other measures told a different story. Attacks on U.S. troops and civilian deaths were at an all-time high by September 2006.

"What did we do? What did we really accomplish?" Da Silva often asked.

He returned home in October and immediately took 30 days of leave to study for the LSAT. Almost 60 percent of Da Silva's West Point classmates left the Army after their initial five-year commitment, one of the highest voluntary attrition rates since the Vietnam War.

MAJ. WALT COOPER

Cooper, the Rhodes scholar, made it to Iraq in summer 2006. His 12-man Special Forces team was assigned to train a battalion of Iraqi national police in Baghdad. In morning formation, Cooper's police sang cadences touting radical Shiite militia leader Muqtada al-Sadr, who vehemently opposed the U.S. presence in Iraq. After dark, the police officers disappeared on raids aimed at ridding Baghdad's few remaining mixed neighborhoods of Sunnis.

"Much of our work is actually counterproductive," wrote Cooper in a final report on the unit. "We are often just enhancing the skills of the very people who contribute to the destabilization in Baghdad."

In March 2007, Cooper and his team returned to Iraq and were assigned to Nasiriyah, a Shiite-dominated city and a key hub for insurgents smuggling weapons from Iran. He teamed up with a battle-scarred Iraqi colonel who was missing part of his right ear and had assembled a 150-man paramilitary unit that nominally reported to the Interior Ministry.

Initially, Cooper dribbled out training and equipment to Col. Abu Liqaa, the Iraqi commander, who maintained a wary distance as well. "It started really slowly at first," Cooper said.

In June, hundreds of fighters from Al-Sadr's militia launched an all-out assault on Nasiriyah. Cooper's team, backed by withering fire from U.S. aerial gunships, rolled into battle with the Iraqis they had been training and routed the militia forces.

The battle forged a bond between Cooper and Abu Liqaa. "We developed an awesome relationship," Cooper said. When Gen. David Petraeus, the top U.S. commander in Iraq, defended his war strategy to Congress in fall 2007 he cited Abu Liqaa's leadership as evidence that Iraqis were taking control of their country. "His forces have stood up to militia extremists and even pursued them beyond Nasiriyah," Petraeus testified.

Four months later, insurgents assassinated Abu Liqaa, a twist that seems to symbolize the messiness of Iraq for Cooper. Today Cooper remains proud of his 2007 successes but is also quick to say that they don't erase the dark days of 2006. "What I saw in 2006 wasn't an aberration," he said. "The lesson I fear we will take from Iraq is that we have figured out a way to impose order or governance on other societies. It is a different brand, but the same kind of hubris that got us into the mess I saw in 2006."

WHERE THEY ARE NOW

Thoreen, who for a time considered leaving the Army and going to business school, spent his last tour in Iraq in 2007. Last month, he returned from a one-year tour as the commander of a 120-soldier infantry company in southern Afghanistan. By November, he will have to decide if he is going to stay in the Army or try civilian life. "I am not sure whether I'll ever be comfortable with the answer," he said.

Da Silva, who had submitted his official resignation in December 2006, pulled his paperwork a few months later. In 2007 the Army's new counterinsurgency doctrine, which focused on protecting civilians from insurgent attacks and promoting local governance, swept through the officer corps. "It was like people were speaking a whole new language," he said. He has spent part of the time since puzzling through the inner workings of Iraq's largest oil refinery, trying to stop the siphoning off of funds.

Cooper is finishing his Harvard doctoral dissertation while teaching governance at West Point's department of social sciences. In three years he will return to a Special Forces job.