So it is no surprise that the city's recent fall to al-Qaida-linked forces has touched a nerve for the service members who fought and bled there.
Some call the news "disheartening," saying it revives painful memories of their sacrifice, while others try to place it in the context of Iraq's history of internal struggle since the ouster of dictator Saddam Hussein in 2003. As difficult as it is to see Islamist banners flying from government buildings they secured, they refuse to accept this as a permanent reversal.
"I'm very disappointed right now, very frustrated," says retired Marine Col. Mike Shupp, who was commanding officer of 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."
"I do not see this as the culmination of the failure of all of our efforts — yet," agrees Earl J. Catagnus Jr., who was wounded by an improvised explosive device in Fallujah and now teaches at a military college. "This is just one battlefield, one city in a host of battles that has been happening since 2003. 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 over that ancient trading and cultural center on the Euphrates River certainly does loom 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, room-to-room 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 battle, likens it to "a thousand SWAT teams going through the city, clearing criminals out."
"These young Marines — 19 years old — went in every building and every room of Fallujah," says Lowry, author of the book "New Dawn: The Battles for Fallujah." ''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. And they did that over and over and over again."
Around 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."
Lowry says Fallujah was "the turning point in the war in Iraq for America." And that is why the al-Qaida takeover is such a bitter disappointment for many.
Former Marine Lance Cpl. Garrett Anderson fought in the second Fallujah battle, where his unit lost 51 members. 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."
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 and scout sniper with 3rd Battalion, 5th Marines, Catagnus was gearing up to go out when insurgents detonated the improvised bomb about 8 feet (2.5 meters) away. Despite a concussion and shrapnel wounds to his face, he never left the line.
Now an assistant professor of history at Valley Forge Military Academy & College in Wayne, Pennsylvania, 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 studying for a doctorate in military history. "For the new generation, it's because everybody keeps mentioning it. And that is the battle that really made a warrior a warrior."
And while he concedes that the battle changed 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.
For some veterans, the reversal of fortunes in Anbar, while unfortunate, is hardly surprising.
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, California, 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. 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."
Dr. Harry Croft, a former Army psychiatrist in San Antonio who has evaluated more than 3,000 Iraq veterans for the U.S. Department of Veterans Affairs, said news reports of the unrest may trigger strong emotions in combat veterans.
"I fear that Fallujah may be the tip of the iceberg," he says. "It may be the start of a number of things that will cause these vets to wonder, 'What was the point?' They may start asking questions like, 'Why did my friends have to die? Why am I the way I am? Why did I have to participate in something when the outcome is not what we see in the movies with the signing of an armistice like in WWII?'"
Some who lost loved ones have similar feelings.
"I'm starting to feel that his death was in vain," says Shirley Parrello of West Milford, New Jersey, whose youngest son, Lance Cpl. Brian P. Parrello, 19, died in an explosion there on Jan. 1, 2005. "I'm hoping that I'm wrong. But things aren't looking good over there right now."
For his part, Shupp, the former colonel, is not convinced that many of those holding sway in Fallujah aren't simply "armed thugs, criminal elements, out there pursuing their own self interests." 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, North Carolina. Associated Press writer Kevin Freking in Washington, D.C., also contributed to this report.
Copyright 2014 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.