As the British government prepares to send its soldiers north to free up the US army to attack Falluja, it is necessary to focus on what this coming onslaught will mean for the city and its people. Falluja is already now being bombed daily, as it is softened up for the long-awaited siege. It has been a gruelling year for its people. First, they were occupied by the US army's 82nd Airborne, an incompetent group of louts whose idea of cultural sensitivity was kicking a door down instead of blowing it up. Within eight months of the invasion, the 82nd had killed about 100 civilians in the area and lost control of Falluja, leaving it to the US marines to try and retake the city last April. After killing about 600 civilians, the marines retreated, leaving the city in the hands of 18 armed groups, including tribesmen, Islamists, Ba'athists, former criminals and an assortment of non-Iraqi Arab fighters said to be led by the Jordanian, Abu Musab al-Zarqawi.

Fallujans have now been offered a choice: hand over the outsiders they dislike (mostly Arabs) who are protecting them from the outsiders they really hate (the Americans), or get blown apart by the world's most lethal killing machine, the US marines. Zarqawi's influence on the resistance has been wildly exaggerated - indeed, many people in Falluja don't even believe he exists, and most find the non-Iraqi Arabs' brand of Salafi fundamentalism at odds with their local Sufi traditions. Today, many Fallujans are tired even of their own mujahideen, but trust the US army even less, and with good reason. Recently, a Bush administration official told the New York Times the bombing was driving a wedge between the citizenry and the non-Iraqi fighters. If, indeed, the civilian population is being bombed for this end, this is a grave war crime.

We have a blueprint for what will happen in the city during the coming attack: Falluja, part one. Like all sequels the next time will be bloodier. Last April I found myself inching across a bridge into Falluja holding an old white T-shirt: in front of me, marines blocking the bridge, screaming at me to go back; behind me, a large group of Iraqis yelling at me to go forward so that they could follow me through the roadblock and rescue their families. After a while, the marines opened the bridge allowing hundreds of women and children to stream out, but stopped the boys older than 16 and men younger than 60 from leaving the city. Preventing civilians from leaving a battle is against the Geneva conventions - although battle doesn't capture what a meat grinder the city had become in that first week of the assault, when the majority of civilian casualties were killed, blown apart by precision, and often inaccurate, airstrikes.

The dead were buried in gardens or in mass graves in the city's soccer field. For three weeks 5,000 marines surrounded the city of 340,000 - think an assault on Cardiff. The marines created a moving front line of humvees and tanks, cutting Falluja off. In the air, helicopters and fighter planes bombed a city without air defences, while unmanned drones circled continuously, looking for targets.

During that first week, I was told by Iraqi fighters that the marines nearly took the city after capturing a lot of rebel ammunition: stockpiles of land-mines and homemade rocket launchers that plugged into car lighters. Oil barrels with distances painted on them lined the streets so the rebels could register mortars. The mujahideen were more than a few foreign fighters and Ba'athists, as the US army had been telling everybody.

Initially, the majority of civilian casualties came from bombing that caused "multiple blast wounds, lost limbs, abdomens blown open," as Falluja's doctors told me. According to the Geneva conventions, force must be proportionate and when these images appeared on Arabic television - dead families stacked on top of each other - it looked anything but proportionate; it looked like mass murder. Against the advice of the marine commanders, the White House ordered a ceasefire. The resistance regrouped, re-supplied itself and fought on.

I made it back into Falluja during the second week of fighting by using fake Iraqi ID. I was accompanied by a translator who told people I was a brother suffering a brain aneurysm. We left Baghdad and drove down roads guarded by guerrilla fighters. The countryside from Ramadi east to Falluja and then to Baghdad was in revolt. We had to pass through resistance lines to get to the marines and then through insurgents to get into the city. It was the marines who were surrounded, not the rebels. This is why the US army needs British troops to free up their soldiers.

The Americans have more than enough troops to attack Falluja, but as soon as they do the area will once more erupt, and it will take everything the Americans have to control the surrounding villages of Habbaniya, Khaldiya and Al Kharma. According to the Iraqi president, Ghazi al-Yawar, there is a good chance that when the marines hit Falluja again, even Mosul, home to three million Sunnis, will explode. Unlike the US army, Mr Yawar knows what he is talking about and understands the way the tribes are grouped in northern Iraq, an intricate web of families that runs through the Sunni triangle. If Mosul is pushed over the edge, holding the north will be like trying to keep the lid on a pressure cooker by hand.

Once we got into Falluja, we were taken at gunpoint to a mosque where we were interrogated by a host of people - former Iraqi secret police and Islamists - before being saved by a friend of my translator's who told us later they were holding 18 hostages in another room. Both hostage-taking and using a mosque as a military base are - like preventing the escape of civilians - against the laws of war. You could hear the occasional shots from snipers, the circling drones, tank fire and mortars. At a clinic, the doctors rolled their eyes at the mention of the mujahideen, but most of their anger was directed at the Americans. The hospital, which lies across the Euphrates, had been cut off from the rest of the city by the marines - another questionable act under the Geneva conventions.

Worse still, the doctors said, several of their colleagues had been shot by snipers along with ambulance drivers, both grave breaches of the laws of war. At this point, most civilians being brought in had head and upper body wounds, most likely from marine snipers. Nothing I saw during the bombing of Baghdad could have prepared me for Falluja under siege. It was as if the marines had been able to cut the city off from the idea of safety itself.

The third time I went into Falluja was during negotiations to hand control of the city over to what became the Falluja Brigade. Mujahideen were busy attaching wires to bombs on street corners, in case the negotiations failed.

Today the city is one giant improvised explosive device. But it is the snipers the people of Falluja fear more than anything else.

I have spent time with both resistance fighters and the US army, and there is no question the marines can take the city. But the US has a developed a habit of winning engagements while losing the war - while breaking the laws of war in the process. This is what Britain's redeployment will help to unleash.

• Patrick Graham is a journalist who worked in Iraq from November 2002 until August 2004 for the Observer, Harper's and the New York Times magazines. He is writing a book about his experiences