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STORY: 20 years on since the Iraq War began,

these violent scenes remain a defining moment of the U.S.-led invasion.

On March 31, 2004, four U.S. civilian security contractors drove into the city of Falluja,

tasked with protecting catering trucks.

Masked insurgents ambushed them using rocket-propelled-grenades and AK-47 rifles and dragged their bodies through the streets.

Reuters journalist Michael Georgy arrived in an hour later.

"The scene was quite gruesome, there were bodies that were being burned, other people stepping on the heads of these corpses, young and old, and I remember in the middle of this I was taking notes trying to understand what was going on, when a boy of about nine years old came up to me and he said, 'We took the others, burnt them and hung them on a bridge, Would you like to see them?' He seemed to me to be, in my mind, how I remember it, a tour guide of death."

The Falluja ambush would prove a defining moment in the war.

Far from being "mission accomplished" – as Bush had declared less than a year earlier – the conflict had only just begun.

(Michael Georgy) "The incident of Falluja was a year after George Bush, out of an aircraft carrier, said 'mission accomplished' to the applause of soldiers. There was nothing more far from the truth. That was a start of absolute hell in Iraq."

In March 2003, U.S.-led forces invaded Iraq,

vowing to end the dictatorial rule of president Saddam Hussein and destroy Iraqi weapons of mass destruction.

No such weapons were ever found.

The city of Falluja, some 30 miles west of Baghdad,

was part of the Sunni Triangle, a central region of mainly Sunni Muslims that had been the powerbase of Saddam Hussein.

"Saddam gained a lot of support in places like Falluja by giving them jobs in the army, jobs in the military, businesses. So when the Americans took over, and there was clear that this was going to change, that the Shi'ite will now become the dominant sect, resentment began then."

Anger had started brewing soon after the U.S.-led invasion.

Any euphoria that accompanied the toppling of Saddam Hussein quickly gave way to fear about sectarian rivalries that Saddam had ruthlessly repressed

and to fury at the mismanagement of U.S. occupation.

The attack on Blackwater security contractors triggered two U.S. military offensives in Falluja, in which the city was besieged, surrounded and pounded.

That heralded not just more attacks on American troops

but a broad insurgency that swelled the ranks first of Al Qaeda and then the Islamic State, miring Iraq in conflict and chaos.

"I often think of that young boy and the violence that day. It was very clear that this region, there is no way that it will be able to stabilize any time soon."

When U.S. combat troops pulled out of Iraq in 2011, the violence did not end.

Within weeks the U.S. had disbanded Iraq's army and left 400,000 soldiers without jobs.

Violence spiraled out across the country in the months and years that followed,

often pitting the minority Sunnis against the majority Shi'ites.

Car and truck bombs, improvised explosive devices, suicide bombers, beheadings, sectarian death squads and torture chambers pushed the death toll of Iraqis and others ever higher.

What was supposed to be a short military engagement in Iraq

ended up being an eight-year war that claimed the lives of more than 4,000 American soldiers, hundreds of foreign troops, and tens of thousands of Iraqi civilians.

For Falluja residents, like tribal sheikh Salman al-Falahi, the scars still run deep,

from the 2003 U.S.-led invasion to Al Qaeda's takeover of the city in 2006

to its domination by Islamic State in 2014.

"When people from Falluja were fired at, killed, and injured. What did America and its allies do? Did they apologize? Did they say they were sorry? Did they take some soldiers and leave? No, they were even more dedicated. They sent us their secret services to the city. Of course, the youth of the city, I was impacted psychologically. Cars came and they intercepted them in the street and killed them. Once they killed them, they threw them out in the streets. The people were boiling. So what did they do? They dragged them and hanged them there."

"The U.S. invasion was supposed to deliver stability and democracy. And instead it was mass chaos for years, and violence, sectarianism, and civil war. So it is very, very sad memories."