Ten years ago, US troops and coalition forces fought their deadliest battle since the Vietnam War when they pushed into the Iraqi city of Falluja to root out Sunni insurgents.
The BBC's Paul Wood, who was embedded with the marines at the time, recalls the fierce fighting and asks what it can tell us about today's struggle against Islamic State.
We were on a rooftop in Falluja. A sniper was in a minaret a couple of hundred yards away.
The sound of battle was all around us but the roof seemed awfully still and quiet.
You couldn't see the bullets. It was more like feeling their presence. We all lay flat, an unseen force pressing down. That force was fear.
The military translator stood and shouted down to the street. A dozen civilians, men and women, were inching forward, waving a ragged white sheet.
"Raise your shirts", he shouted to the men. He was afraid they were hiding suicide belts. "OK. Move!" he yelled. "Get the hell away from here: it's dangerous."
We crawled off the roof and fell down the stairs, back into the noise and confusion of the building.
Two marines were at a window, one firing, one spotting. "Right there! Right there!" An hour later we heard that the last man off the roof had been killed. A bullet from the sniper hit him in the back, below his flak jacket, as he jumped into the stairwell.
That was Lt Dan Malcolm. He was 25. I remembered him as a quiet and thoughtful young officer who liked to play chess. Like us, he had left the roof, the danger all too apparent.
He went back because of a desperate call from another lieutenant, Elliot Ackerman, whose platoon was coming under friendly fire.
"We had these artillery rounds landing in the street in front of us," Lt Ackerman told the BBC last week.
"I could hear the steel slapping against the building we were in. So I got on the radio, screaming out, trying to figure out what was going on. Dan ran back up to that rooftop to see where the rounds were landing and call them off of us."
'Seize the city'
Ten years on, Elliot Ackerman, deeply affected, still wears a wristband to remember Lt Malcolm.
How was it that the biggest battle of the Iraq campaign was fought more than a year-and-a-half after the invasion - and after the now notorious declaration by President Bush of "Mission Accomplished"?
Falluja was never going to welcome the Americans as crowds of Shia Muslims did when US forces arrived in Iraq, in March 2003.
Falluja was Sunni - and it had done well under Saddam Hussein's Baath party rule. Its sons joined the army and the police. After March 2003, they were unemployed.
Some turned to crime: the highway near Falluja became notorious for armed hijackings. Some joined the "resistance".
On 31 March 2004, four American private security contractors were ambushed in the centre of Falluja. They were probably killed when their armoured four-wheel-drive vehicle was hit with a rocket-propelled grenade.
To make sure, small boys brought jugs of petrol and a crowd set the bodies alight. The men's charred bodies were hung from a bridge over the Euphrates.
The US Marines wanted to move into Falluja "like a soft mist" and quietly arrest the guilty, said the military historian Bing West, a former Marine Corps officer in Vietnam.
As the crisis unfolded, Mr West was at the headquarters of the marines' commander, Gen James "Mad Dog" Mattis.
"Ambassador [Paul] Bremer [former US administrator in Iraq] and President Bush got very angry and emotional and the order came down the chain of command, 'No, you are to seize the city,'" Mr West told me in an interview in 2006.
"And the marines said, in a polite way, 'Do you know what it takes to seize a city of 300,000?'"
'Noose around Falluja'
But that was exactly what the US Marines were ordered to do. Their attack in April 2004 - the first battle of Falluja - quickly got bogged down. Marines I spoke to recalled being trapped, fired on from all sides as the young men of Falluja rose up to defend their city.
Civilian casualties were inevitable and anger spread across Iraq, leading to street protests and more attacks on coalition forces.
There was unrest among the Shia majority, not just among Sunnis. Members of the US-appointed Iraqi Governing Council threatened to walk out. Iraq was boiling over. The marines were ordered to retreat.
After that, Falluja became a safe haven for al-Qaeda. Kidnap victims - Iraqi and foreign - disappeared there. Strict sharia [Islamic law] was enforced. Locals spoke of beheadings in the street.
Outside, a long, slow military build-up began. Civilians were warned that they should leave. A noose was thrown around the city. Anyone suspected of being an insurgent was arrested at checkpoints.
In October 2004, journalists came to Camp Falluja to "embed" with the force that would shortly try to retake the city.
During the first battle, in April, Muslim opinion had been outraged by US targeting of mosques. Fallujah has so many it is known as "the city of mosques".
They were being used as insurgent bases, the minarets sniper positions. The marines wanted journalists there to witness that.
I joined 1st Battalion, 8th Marine Regiment - the 1/8 - led by Lt Col Gary Brandl.
Col Brandl told his men they were leaving behind the shadow war of insurgent ambushes and roadside bombs planted by a "faceless enemy".
"The enemy has got a face," he said. "He's called Satan. He's in Falluja. And we're going to destroy him."
Years later, that is still one of my most memorable quotes of the Iraq war. The unit's pastor said the "wrath of God" would be called down on the "terrorists, evildoers" in Falluja, with the US Marine Corps the instrument of that wrath.
The night before everyone "stepped-off", Col Brandl explained the battle plan to his officers, using a mock-up of Falluja laid out on the dusty floor of a barracks.
One rifle company would take the main road into Falluja. A second would go up that road to move into the complex of government buildings in the centre.
It was an old fashioned marine charge. "Hey diddle-diddle, straight up the f-ing middle," one of the officers said to me quietly.
So as the sun rose a couple of days later, the US forces in the city centre buildings found themselves under fire from all sides.
The fighting was at close quarters, with the two sides sometimes just a few yards apart
Meanwhile, Col Brandl strode around like a Hollywood version of what a Marine Corps officer should be, cigar stuck between his teeth as he dished out orders.
The bodies of insurgents lay in the streets for days, being gnawed at by dogs.
We moved to a house with one squad. The owners had left behind Baath party membership cards and pictures of Saddam Hussein.
Outside, insurgents were hiding behind a breeze block wall. As night fell, they fired rocket propelled grenades that made yellow streaks as they sailed over the house.
Marine snipers using thermal imagers could see the insurgents and shot them through the wall.
The marines lost a man in an intense firefight for another house that went on most of the morning. Inside, they found the bodies of two men and a boy aged about 10. The whole squad was very upset about that.
To me, it seemed to show that some of the menfolk of Falluja had stayed behind to fight the Americans.
The marines were not just facing the foreign jihadists whose passports - Saudi, Egyptian, Algerian - US commanders said had been found on insurgent bodies.
But most of the people had left Falluja. The only civilians we saw were on the first day, from the roof of the building where Lt Malcolm was killed.
That was the crucial difference with the first battle of Falluja. The image of a city packed with non-combatants being pounded with artillery and white phosphorous was wrong.
After a week of combat, the marines had taken back Falluja. The 1/8 alone had lost more than 20 men.
A year later, we went back to Falluja. A Marine Corp major showing us around stood embarrassed as a woman in black niqab screamed at him that her husband had been shot dead at a checkpoint.
As we found out later, the Iraqi Army, not the Americans, had killed him. But people were angry at the US.
That didn't change until the Sunni tribal "Awakening" - and only then because Sunnis feared genocide at the hands of Shia death squads.
Now the black flag of jihad flies over Falluja once again, the Islamic State in control.
Sunnis have turned to them because of hatred of the Shia-led government in Baghdad.
American military advisers are back in Iraq but the US is not about to refight the battle of Falluja.
The lesson of 10 years ago for Western governments battling the Islamic State today? It is that military force alone is not enough for lasting victory.