For American veterans of Fallujah, seeing Al-Qaeda militants regain control of the Iraqi city is a painful sight, leading some to question what they were fighting for nearly 10 years ago.

The bloodiest combat of the US war in Iraq took place in Fallujah in two intense battles in 2004, with US Marines rolling back militants in days of brutal street fighting.

But now Al-Qaeda flags are flying over Fallujah in western Anbar province after extremist gunmen swept into the city last week.

Their gains illustrate how security is unraveling in Iraq, two years since US forces pulled out of the country.

"It's a bitter pill to swallow," said David Bellavia, a retired Marine staff sergeant who was awarded the Silver Star, one of the military's most prestigious combat decorations, for his role in the battle.

"This administration has decided Iraq is not important," he told a local blog in his hometown of Batavia in New York state.

Some Republican lawmakers, including Fallujah veteran Duncan Hunter argue that the violent extremists could have been contained if President Barack Obama had pushed harder to keep some US troops on the ground in Iraq.

"I think the overwhelming feeling is anger," said Hunter, who served with the 2nd Battalion, 1st Marine Regiment in Fallujah.

"The administration let our victories count for nothing."

Other veterans were more philosophical, saying it was never clear what American forces were doing there in the first place and it was up to Baghdad to secure Iraqi cities.

"Over the years looking back on my two deployments to Iraq, I have wondered if there was something from there that was worth all the fighting, death and destruction.

"I haven't found that yet," said Chris Garrett, writing on a Fallujah veterans Facebook page.

Throughout the Iraq conflict, Fallujah and the Anbar region stood out as a hotbed of insurrection.

In April 2004, the death and mutilation of four employees from the private security firm Blackwater led to a US offensive against insurgents in Fallujah.

The American troops were accused of employing harsh tactics and causing excessive civilian casualties in their fight with militants.

Coalition forces later handed over control to local Iraqis in the city but insurgents gradually reasserted themselves, leading to the second battle of Fallujah in November-December 2004.

More than 10,000 US Marines took the fight to the insurgents in what was codenamed "Operation Phantom Fury," backed up with massive artillery fire as they rooted out militants block by block.

The cost was high, both for the US troops and Iraqi civilians killed in the fighting.

"It's heartbreaking to see the deterioration of the situation in Iraq, especially given the many sacrifices of US service personnel to give Iraqis a chance for a better future," said Peter Mansoor, a retired colonel who served as executive officer to the former commander in Iraq, General David Petraeus.

After Washington sent in reinforcements in 2007, US forces "were the glue that held Iraq together" and "were seen as honest brokers by most Iraqi sects and factions, especially the Sunnis who had partnered with US troops to fight Al-Qaeda," Mansoor said.

The withdrawal of US forces from Iraq was a mistake that opened the way for extremists to resurge, he said.

Fellow "Phantom Fury" veteran Theodore Lester shared a similar view, saying the fall of Fallujah was inevitable.

"We broke it, taped it back together, then claimed it was brand new as we threw it down and walked out the door," Lester wrote on Facebook.

"This is the beginning of a new civil war for them. My only surprise is that it didn't happen sooner."

Senator John McCain, a Vietnam veteran and an outspoken critic of Obama's handling of Iraq, said he was appalled but not surprised by events in Anbar.

"The fact is, there were 95 soldiers and Marines killed in the second battle of Fallujah, over 600 wounded, and now that all, of course, is wasted," McCain told AFP.

But even Obama's fiercest critics on Iraq are not calling for sending American soldiers back into the country.

It may be to difficult to watch Al-Qaeda gaining a foothold in a city where so many Americans fought and died to keep them out, but the time for combat has passed, veterans said.

"Operation Phantom Fury was a success, we took a full-fledged city of insurgents and gave 'em hell," Eric Hernandez wrote on Facebook.

"We won and did our part... It's not our problem anymore so they can keep it."