World view - Lindsey Hilsum watches troops prepare for Fallujah

The wall between the US marines' culture and the Iraqi world is almost impenetrable. The troops cann

For the past week, I have been "embedded" with the 1st Marine Expeditionary Force, in advance of the US army's promised attack on Fallujah.

The marines use mainly nautical language, so I didn't arrive: I "came on deck". I don't go to the toilet, but to the "heads". The rest of the jargon is military. I eat not in a canteen, but in a "chow hall". I'm not all right, but "good to go". I have been out in an AAV, watched them set up a VCP, and become rather nervous about the likelihood of a VBIED.

I have accompanied patrols along "Michigan" and "Mobile", with marines observing the battered buildings on either side of the route through the sights of their M-16 rifles.

Those highways are familiar from many trips during the past two years, when I have come to know that "Michigan" is simply the old main road from Baghdad to Jordan and Syria. Renaming isn't an imperial plot. It is done for the marines' own communication, so that all of them can remember. In theory, it doesn't affect anyone else, but I am beginning to feel that their way of placing the American template across the map is the essence of the problem.

The amphibious assault vehicles (or AAVs) look threatening, and most Iraqis now know to stop at the vehicle checkpoints (VCPs). One problem is that, if anyone decides to ram through with a vehicle-borne improvised explosive device - what you or I might call a suicide car bomb - it is likely to take out the other waiting cars as well as the marines.

Just east of Fallujah, our unit received a message that groups of fighters were believed to be heading for the city in small convoys of four-wheel-drive vehicles. All men of military age were to be arrested.

The first car the marines stopped was carrying a family who said they were heading for the holy sites in Syria. The women, swathed in black, climbed out carrying the children while the men were pulled to one side. The mother tried to proffer suitcase keys, but the marine either didn't see the gesture or failed to understand, and knocked off the locks with a zap-stick.

In his wallet, the father of the family was carrying a picture of Ali, the Prophet Muhammad's son-in-law, the equivalent of a saint to Shia people, but a figure of no significance to Sunnis. The fighters in Fallujah are Sunni Muslims, so this was an important clue.

"Why's he got a picture of Jesus?" asked a marine.

Heading the other way was a set of vehicles carrying merchants bringing in goods from Syria and Jordan. One car had several dozen old-fashioned tape recorders still in their boxes, scores of cassettes and some flyers for a new album called Assala's Sadness. The other was loaded with bales of cheap, knock-off football shirts - red-and-white, all red, green-and-white, yellow-and-black.

"Hey, that guy's got loads of uniforms," said a marine.

While the vehicles were being searched, the men were lined up on the verge for the kind of intrusive body search which, to marines, is normal practice. Each man is made to face away from his car as it is being checked. His hands are placed on his head and his legs kicked apart so that the marine maintains the advantage of balance as he quickly pats down the whole body. There is no abuse here, and it is undoubtedly an effective form of body search, but the experience is aggressive and inevitably humiliating.

For an agonising hour, it seemed that everyone was going to be arrested. When the explosive ordnance disposal unit carried out a controlled detonation a few hundred yards away, the women burst into terrified tears and the children started wailing.

Unable to contain ourselves, my companion explained the football shirts and I pointed out one man's Lidl shopping bag. This, combined with his perfect English, suggested that he was, as he said, an Iraqi living in Britain and taking presents to his family in Baghdad.

Yes, it could all have been an elaborate disguise, and first impressions are not everything, but it helps if you know what you are looking at. The marines were lost in translation, not of words, but of concepts and knowledge. Their translator turned Arabic into English, but couldn't explain all the little signs that indicate whether someone really is a pilgrim or a merchant, not a jihadi.

Each marine is given a laminated "Iraq Culture Smart Card" explaining key cultural points: "Don't point with a finger - it is a sign of contempt"; "Appear relaxed and friendly - social interaction is critical in building trust." The most useful phrases of Arabic are written out phonetically in English script - "lower your hands", "do not resist" and (probably less usefully) "Do they have chemical, biological or nuclear weapons?"

But the failure to understand what they are seeing goes much deeper, because the wall between the marines' culture and the Iraqi world is almost impenetrable.

Eventually, common sense prevailed. Everyone was allowed to go on their way, apart from 11 men who had been travelling together and had no real cover story. They were blindfolded and loaded into the AAV, to be transported to a detention centre for further questioning. Five of them were released that same day.

By the time you read this, the battle for Fallujah may have begun. The US forces gave notice so civilians could flee. Presumably, the insurgents can flee, too, but Fallujah has become a symbolic target: the Americans have to win it back so that the interim Iraqi government can prove its writ runs there. Some lessons may have been learned from last year - the Americans now know that winning the war is easier than winning the peace.

"The day after in Fallujah is being prepared in just as much detail as the day before offensive operations," said Colonel Mike Shupp, commander of Regimental Combat Team 1. After the Americans retake the city, the Iraqi authorities are meant to start work at once on creating local government, distributing food and rapidly restoring water and electricity supplies.

At the PX on the base (the receipts read: "Serving the Best Customers in the World!!!"), you can buy anything an American might need - Oreo cookies, CD players, peanut butter - and some others that no one really needs, such as a T-shirt bearing the legend, "Where's Your Bagh-Daddy?" Next door is a small gift shop retailing roughly carved wooden camels and saccharine paintings of women in chadors and old men in red-and-white-chequered dishdashas. The marines can buy their souvenirs there - representations of an imagined Iraq, one that you can see without demolishing the concrete blast-proofing in your mind.

Lindsey Hilsum is Channel 4 News international editor