I looked into the future in Iraq, and it wore a long black veil

Every single pilgrim said Britain and America should get out of Iraq quickly or they would face a Shia revolt

They moved south in a long, unbroken line. The dust rose in little clouds beneath each marching group so that the effect was of an early summer sandstorm. Except this was a cloudless and windless day. The dust rose to the knees of the marchers and hung there so that at times they looked like spirit people, suspended above the ground. Stopping by the roadside you could hear their prayers and chanting, and below it the soft beat of sandals and bare feet.

I tried to think of comparisons. Mahatma Gandhi's march to the sea in 1947; the marches to Mandela's first rallies in South Africa, the crowds on Tiananmen Square or at the Berlin Wall in 1989. But this scene on the plains south of Baghdad had a complex power, and it felt very ancient. I will never see anything like this again, I told myself.

How many were there? I hate the numbers game, all fitted up to prove or disprove according to your particular bias. But it's worth talking numbers here, for they spoke, at that particular moment, of raw emotion unleashed on a scale that the Middle East has not seen since the heyday of Nasser. I gave up counting. I was in Karbala for three days and the crowds kept pouring into the square. At two, three and five in the morning the new arrivals woke me with their chanting. Was it one million or two or three? We will never know but my money is on the top end of the scale.

This week in Karbala the world began again for the Shia of Iraq. An oppressed majority heard the call of their leaders and answered with their feet. There is so much to remember: the faces of ordinary Shia murdered by Saddam after the last Gulf war on black and white posters all over town; the crowd dancing on the overturned statue of Saddam; the stalls dispensing free food and drink, every house in the city opening its doors to tired pilgrims; the young men with their assault rifles discreetly tucked by their sides patrolling the outskirts of town; the absence of any police or military and still the extraordinary discipline; the women universally dressed in the black chador.

I looked into the future in Karbala this week and it wore a long black veil. It quoted Islamic law and it was willing to follow the religious scholars in peace and war. I was never once abused for being Western; indeed, many of the pilgrims made a point of saying thanks for Britain's role in driving Saddam from power. But every single pilgrim said Britain and America should get out of Iraq quickly or they would face a Shia revolt. Were all those men and their leaders indulging in impassioned rhetoric when they spoke of creating an Islamic state? I don't think so.

The vast crowds we saw in Karbala represented the ranks of the dispossessed and abused. After the brutal lie that was the Saddam regime, small wonder the Shia are ready to embrace the pure truths offered by the imams. After the law of the monster would not Sharia law seem an attractive alternative? The Iraq of Saddam was a republic of arbitrary cruelty. The powerful took from the weak. They stole their money, houses, women, whatever pleased them. The imams have promised to change all of that.

One story to illustrate the point. An Iraqi colleague was reporting a Shia demonstration near the Palestine Hotel this week when his pocket book was stolen. It contained some money, his identity card and a few family photographs of sentimental value. Afterwards he complained to one of the stewards. The following morning the imam who had organised the demonstration arrived at the hotel to return my friend's pocket book and to announce that the miscreant who committed the theft had been chastised. "After that I would vote for them," my colleague said.

There are echoes of different groups in this Shia movement. For organisational skills they are not unlike Sinn Fein; in terms of marrying faith and politics they are like Hizbollah in Lebanon. Like both Sinn Fein and Hizbollah they have been quick to grasp the importance of social-welfare work in the community. Most important they are on the ground talking to people every hour of the day. People have not forgotten who was with them during the long darkness of Saddam.

How did the Pentagon think they would accept a sleek and well-groomed exile like Ahmed Chalabi? Ahmed Chalabi and the other pretender to Iraq's throne, Hamad Zobeidi, sit in their hotel rooms dreaming dreams of power. Out in the Shia neighbourhoods they are already sharpening the knives for these two returnees. The emergence of this powerful Islamic movement took us all by surprise. It is a failure of knowledge and understanding for which we may now pay dearly.

I only realised the hour of the imams had come the day we arrived in Baghdad, about 24 hours after Saddam fall. In the Shia neighbourhoods young men with green arm-bands and assault rifles manned the roadblocks and politely directed us towards the local religious leaders. A day or so later we heard how the clerics had issued fatwas against looting and that stolen goods were being returned to mosques. It is by no means a monolithic movement with one leader and one plan. There are factions within the Iraqi Shia movement which are quite capable of turning on each other in the fullness of time.

For all the organisational ability of the different factions there is still a lot of political naivety. Some of the imams know that they will have to deal with the immense power of Washington if they are to have a stable Iraq; others believe a jihad will drive the Americans out and deliver a devout and successful Islamic state. Wrong. It will deliver an independent but even more war-ravaged and impoverished state with the prospect of civil war with minority groups such as the Kurds and the Sunni.

A lot depends now on how the Americans respond to the growing Islamic movement. If they try to crack down, they will face a guerrilla war and will eventually lose. They should realise too that this movement has a distinctly nationalist tone: don't overplay the role of Shia Iran in the growing militancy inside Iraq. The idea of Iraq becoming a theocracy ruled under strict Islamic law is scary for a lot of reasons, not least because the Iraqi people as a whole have lived for far too long with the imposition of absolute truths by an unquestioned elite. I mistrust anybody who promises me Utopia; it always ends up being built on skulls and bones.

But there degrees of Islamic state and the Americans need to show they are not scared of engaging with the clerics and the political groups around them. There are signs that some on the American side and certainly the British realise this. Yet as I write I have a feeling I might be the naive one. Perhaps the moment of logic has passed, missed in the run-up to war when the world was convulsed by diplomatic mud-wrestling at the United Nations, and ignoring the most pressing question of all: who would rule Iraq when the monster was, inevitably, driven from power.

The writer is a BBC Special Correspondent