But while many are disheartened at Fallujah's recent fall to Islamist forces, others try to place it in the context of Iraq's history of internal struggle since the ouster of dictator Saddam Hussein in 2003. And they don't see the reversal as permanent.
"I'm very disappointed right now, very frustrated," says retired Marine Col. Mike Shupp, who commanded the regimental combat team that secured the city in late 2004. "But this is part of this long war, and this is just another fight, another battle in this long struggle against terrorism and oppression."
Former scout sniper Earl J. Catagnus Jr. fought and bled in the taking of that ancient city on the banks of the Euphrates River. Now a military historian, Catagnus feels the battle has taken on an almost disproportionate importance in the American mind.
"If you watch 'NCIS' or anything that has a Marine ... they always say, 'Oh, I was in Fallujah,'" says the Purple Heart recipient, who left the military as a staff sergeant in 2006 and is now an assistant professor of history at Valley Forge Military Academy & College in Wayne, Pa. "For the new generation, it's because everybody keeps mentioning it. And that is the battle that really made a warrior a warrior. ...
"It's just for us as Americans, because we've elevated that battle to such high standards ... that it becomes turned into the 'lost cause,' the Vietnam War syndrome."
In the annals of the Marine Corps, the battle for Fallujah looms large.
The fighting there began in April 2004 after four security contractors from Blackwater USA were killed and the desecrated bodies of two were hung from a bridge. The so-called second battle of Fallujah code-named Operation Phantom Fury came seven months later.
For several bloody weeks, the Marines went house-to-house in what has been called some of the heaviest urban combat involving the Corps since the Battle of Hue City, Vietnam, in 1968. Historian Richard Lowry, who interviewed nearly 200 veterans of the Iraq battle, likens it to "a thousand SWAT teams going through the city, clearing criminals out."
"They entered darkened rooms, kicking down doors, never knowing if they would find an Iraqi family hunkered down in fear or an Islamist terrorist waiting to shoot them and kill them," says Lowry, author of the book "New Dawn: The Battles for Fallujah."
About 100 Americans died and another 1,000 were wounded during the major fighting there, Lowry says, adding that it's difficult to overstate Fallujah's importance in the Iraq war.
"Up until that time, the nation was spiraling into anarchy, totally out of control," says Lowry, a Vietnam-era submarine veteran. "The United States Marine Corps with help from the Army and from the Iraqis went into Fallujah and cleared the entire city and brought security to Anbar Province, allowing the Iraqis to hold their first successful election."
And that is why the al-Qaida takeover is such a bitter disappointment for many.
Former Marine Lance Cpl. Garrett Anderson's unit lost 51 members in the city. When he considers whether the fighting was in vain, it turns his stomach.
"As a war fighter and Marine veteran of that battle, I feel that our job was to destroy our enemy. That was accomplished at the time and is why our dead will never be in vain. We won the day and the battle," said the 28-year-old, who now studies filmmaking in Portland, Ore. "If Marines were in that city today there would be dead Qaida all over the streets again, but the reality is this is only the beginning of something most people who have been paying attention since the war began knew was going to end this way."
On Tuesday, the site duffelblog.com posted a satirical column about two former Marines raising $1,300 on Kickstarter to go back and retake the city in time for the battle's tenth anniversary.
"We paid for that city and we're keeping it!" one fictional commenter tells the site.
The piece had more than 30,000 Facebook likes by Wednesday.
Lowry says the U.S. "abandoned" the region's Sunnis, paving the way for a Shiite-led government that has "gotten into bed with the Iranians." He adds: "There is a polarization returning between the Shiites and the Sunnis ... and it's spreading."
Catagnus and others say the situation is more nuanced than that.
A sergeant with 3rd Battalion, 5th Marines at the time, Catagnus was gearing up to go out when insurgents detonated the improvised bomb about 8 feet away. Despite a concussion and shrapnel wounds to his face, he never left the line.
While conceding that the battle helped change doctrine for urban warfare, he thinks Fallujah has become politicized especially here at home.
"There's a lot of fiery language around it," he says. "I do not see this as the culmination of the failure of all of our efforts yet."
Roman Baca, who served in Fallujah for about eight months as a sergeant in the Marine Corps Reserves, says it's hard for him to hear people question the military's work there. During his time, his machine gun platoon spent many of its days patrolling local villages, delivering school supplies to students and food and water.
The 39-year-old New York City man returned to Iraq last year to conduct a dance workshop. He's most worried about what the outbreak of violence means for the Iraqis.
"You think of those kids in the villages that were so young who are now either teenagers or in their 20s," he says. "What does it mean for them? What does it mean for the interpreters who were in danger then and are in danger again because they helped the Americans and their cause?"
For some veterans, the reversal of fortunes in Anbar, while unfortunate, is hardly surprising.
"I was always of the impression that Iraq was sort of doomed to fail no matter what we did," says Derek Richardson of Redmond, Wash., a former Marine corporal who fought in the house-to-house action in late 2004. Now an investigator in Microsoft's legal department, he adds, "For me, it was more about winning individual battles" and keeping his comrades safe.
David R. Franco survived a roadside bomb blast outside Fallujah in 2005. The retired Marine suffers from back pain, traumatic brain injury, post-traumatic stress disorder, and other ailments that send him to doctors and psychologists regularly.
"To me, it was just a matter of time for it to happen again and for al-Qaida to go back in there," said the 53-year-old veteran of Moorpark, Calif., who retired as a sergeant major. "It'll be a constant thing." Still, Franco whose son was also wounded in Iraq says it was worth it.
So does Nick Popaditch.
On April 7, 2004, Popaditch's tank was struck by a rocket-propelled grenade as he rolled through the city. Shrapnel tore through his sinuses and destroyed his right eye now strikingly replaced by a prosthetic bearing the Marine Corps logo.
The gunnery sergeant's actions earned him a Silver Star and Purple Heart, but cost him his career. The San Diego-area man is studying to be a high school math teacher, and he refuses to second-guess the recent events in Iraq.
"There's a lot of downtrodden people there who got a shot at a free life, at freedom," says Popaditch, 46, who ran unsuccessfully for Congress in 2012. "And if the bad guys come back into control, that's not something I can control 8,000 miles away here. I'm just proud of the fact that when it came time to stand and fight for those things, those concepts of freedom, liberty, human rights ... I'm glad my nation did it."
For his part, Shupp, the former colonel, is convinced that many of those holding sway in Fallujah aren't al-Qaida, but simply "armed thugs." Even before the U.S.-led invasion, many Iraqis considered the city a "crossroads of criminal activity," and his troops were never meant to be "an army of occupation."
"It's one of the lifetime struggles of good versus bad," says Shupp, who now works as a defense lobbyist in Washington, D.C. "And this is the time for Iraq to come forward. We gave them all the tools. We gave them the ability to fight these guys."
Watson reported from San Diego. Breed, a national writer, reported from Raleigh, N.C. Associated Press writer Kevin Freking in Washington, D.C., also contributed to this report.