THE debate over whether to send British troops to central Iraq is a sideshow to a much bigger and potentially more momentous drama.

If the Black Watch does relocate to areas south of Baghdad, it will be to free up US Marines and soldiers for the assault on Fallujah, the most violent and dangerous city in Iraq and the de facto capital of the insurgency.

Since the invasion 18 months ago, the city of 300,000 people has been transformed from a place of restiveness and protest to a threat to the entire US deployment in Iraq. Coalition military officers speak of it as a “cancer” eating away at their efforts to impose stability, or a “virus” compromising their control of all of west-central Iraq.

Apart from the large numbers of Iraqi mujahidin based in the city, Fallujah is one of the bases of the foreign fighters who are entering Iraq from Jordan, Syria and Saudi Arabia.

Last week, Iyad Allawi, the interim Prime Minister, demanded that Fallujah hand over Abu Musab al-Zarqawi, the Jordanian militant. It was this bold demand which has served as the trigger — or the pretext — for the Ramadan offensive against the city, which began last Thursday.

The city enjoyed relative calm yesterday in the wake of fierce gun battles on Sunday, in which at least four civilians — including a child — were killed and 12 wounded. US forces released Fallujah’s chief negotiator whom they had detained on Friday as street fighting and air strikes rocked the city.

US officials continue to insist that the fighting is simply an intensification of long weeks of bomb attacks against Fallujah, and deny that it amounts to a new attempt to recapture the city. They are wise to avoid raising expectations as the last time an all-out assault was ordered last April, it ended in failure and humiliation. It was widely seen as an ill-judged response to a shocking act — the murder and mutilation of four Americans who were taken in Fallujah while driving to Baghdad. The subsequent assault by US Marines was opposed by their regional commander, Lieutenant General James Conway. But the Marines were even more appalled when Lieutenant General Ricardo Sanchez, the former commander of US forces in Iraq, ordered a withdrawal half way through the attack. The reason appears to have been pressure from Iraqi politicians wary of the damage to their own standing caused by inevitable civilian casualties.

The intensive bombardment and the deaths of women and children as well as guerrilla fighters had already radicalised many formerly uncommitted inhabitants of the city. And the compromise solution imposed by General Conway made the situation even worse. He formed a unit called the Fallujah Brigade, made up of former soldiers from Saddam Hussein’s army. At best, it was completely ineffective in imposing central government authority on the city; at worst, its members actively collaborated with the very guerrillas they were supposed to police.

Since then the city has effectively become an independent fiefdom, uneasily ruled by a council of resistance leaders, clerics and politicians. Worse, it has become a magnet for anti-American fighters from all over Iraq and the wider Middle East. Many of the suicide bombs which have exploded in Baghdad are believed to have come from Fallujah. During the April encirclement of the city, it was later noticed that such attacks had ceased during that period. Several of the foreigners and Iraqis kidnapped and beheaded by Islamic groups met their bloody end in Fallujah.

The democratic elections scheduled for January — the coalition’s one hope now for a happy ending to its invasion of Iraq — will be seriously compromised if they cannot be held in Fallujah, and in the restless cities adjacent to it, such as Ramadi. But retaking the city is a daunting military task.

Compared to the fighter jets, helicopters, artillery and tanks of the US Army and Marines, the guerrillas are poorly armed but they have the immense advantage of defending an urban environment, with homes, schools and mosques from which to attack their invaders.

It would be surprising if President Bush was to risk such a potentially costly enterprise in the run-up to the US election. Many expect that a major assault will come afterwards.

But whenever it comes, the stakes in Fallujah could not be higher. Success will, temporarily at least, cut off an arm and a leg of the resistance. Failure will result in a further devastating loss of confidence in the coalition presence in Iraq.

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