American forces advancing on Baghdad were yesterday fighting on the outskirts of one of the holiest places in Shia Islam. The 101st Airborne Division surrounded the holy city of Najaf, and there was talk of it preparing for possible house-to-house fighting, a move that would run the risk of inflaming the Shia world.
The US military said it had killed 100 Iraqi paramilitaries around Najaf, and said more were lying in wait among the tombs of saints and martyrs in the great Wadi al-Salaam graveyard which encircles the city on three sides, one of the world's largest cemeteries.
Further north, US soldiers were fighting what was reported to be the most intense ground battle of the war so far, in Hindiyah. But on the road ahead of Hindiyah, between them and Baghdad, lies a Shia shrine of perhaps even greater symbolic potency, the holy city of Karbala.
British and American soldiers and their commanders have been perplexed and dismayed that the Shias of Iran, persecuted and repressed by Saddam Hussein, have not yet, at least greeted them with flowers as liberators, or rebelled against President Saddam's forces, as they were expected to do. No one is entirely sure why they have not.
But what happens around Najaf and Karbala in the coming days may turn out to be the key to how the British and Americans are perceived by the Shia. The American troops surrounding Najaf, the resting place of Ali, the son-in-law of the Prophet Mohammed, appear to be aware of that. Many sites in the city have been declared "no targets", to be fired at only in self-defence.
American soldiers drove around the outskirts of the city yesterday, beseeching its inhabitants through loudspeakers mounted on their armoured cars to turn over President Saddam's forces to them, so wary are they of trying to face Iraqi fighters in the streets of Najaf. But US commanders said yesterday there were too many fighters in Najaf to ignore.
Karbala, the resting place of the Prophet's grandson, Hussein, is perhaps even more dangerous for the Americans and the British. So holy is the city to Shia Muslims that many carry soil from Karbala to prayers with them, even in faraway cities in Iran, cut off from Karbala since the Iran-Iraq war.
Others sleep with tablets made from the earth of Karbala under their pillows. Some Shias regard a pilgrimage to Karbala as more important than the haj to Mecca and Meidan, which is among the requirements of Islam.
The fall of the two holy cities would be a blow to President Saddam, but it would be a blow he would try to turn to his advantage. Any damage to the great gold-domed shrines of the two cities could turn the Iraqi Shias against the British and Americans, and cause fury across the Shia world.
More than that, the two cities are shrines to martyrs. The concept of martyrdom is at the heart of Shia Islam, and none is more laden with significance than the martyrdom of Hussein at Karbala. Any large-scale casualties in these holy cities would be charged with extraordinary symbolism.
There has been much speculation on why the Shia of Iraq have not welcomed the Allies as liberators, or risen up against President Saddam's forces. American and British leaders have suggested that it is because the Shia are still afraid; most, outside a few captured small towns such as Umm Qasr, are still under the barrels of his guns.
Many in the Arab world say the Shias are Iraqi nationalists first and will put resisting invasion and occupation by British and American forces above their hatred of the dictator.
There is also the shadow of the Shia uprising against President Saddam after 1991's Gulf War. Many Iraqi Shias felt betrayed after George Bush senior urged them to rebel, then failed to come to their aid. President Saddam, his hands unfettered by the Americans, took a terrible revenge on the Shias, killing and torturing by the thousands.
The US has had an uncomfortable relationship with Shia Muslims since the 1979 Iranian Revolution, a Shia revolution, when revolutionaries blamed the US for supporting a coup by the Shah of Iran against the country's first democratic government and US diplomats were held hostage for a year.
One of the most important Shia Iraqi opposition leaders, Ayatollah Mohammed al-Hakim, has sent instructions to his supporters in Iraq neither to fight against the Americans and British nor to rise up.
Ayatollah Hakim, who controls a force up to 30,000 strong, is living in Iran, where the regime may be influencing him. He has warned the US not to try to stay on in Iraq after the dictator has been toppled.
Now US forces are at the gates of the two holy cities. Karbala and Najaf are at the heart of the identity of Shia Islam, and its differences from Sunni, or orthodox Islam. Their significance has not been lost on President Saddam.
Although he is a Sunni Muslim and was, in his early years, overtly anti-religious he has made great efforts to ingratiate himself with the Shia minority by praying at the mosque in Karbala and giving large amounts of cash to it over the years.
Since the Eighties, he has seen it as a vital element in keeping the Shias loyal. Then, as now, it was assumed they would be open to rising against him, because of his Sunni background and because the Baathist state he established was profoundly secular.
It was to avoid this that Saddam played the Islam card and developed public shows of piety, ensuring he was filmed praying in public, particularly at Shia shrines like Karbala.
During the Shia 1991 revolt, the rebels briefly took control of Najaf and Karbala. The vast Wadi al-Salaam cemetery is testament to how many Shias want to be buried close to it. The Shias believe the leadership of the Islamic world should have passed to the Prophet's descendants, through Ali's line, not to the Caliphs who took control. The fate of Ali's son, Hussein, who was martyred in a massacre by the soldiers of the Ummayad caliphate in the year 680, was an event of mythical proportions in the Sunni-Shia split, which defined Shia piety as one of protest and suffering. Hussein insisted on facing the Sunni Ummayad leader, Yazeed, in battle, despite knowing that he would be killed in the clash.
Mourning for Hussein, in which religious Shias whip and cut themselves, some until they are covered with blood, is one of the most important events in the Shia calendar.
The Shias have traditionally been a downtrodden minority within Islam and, generally speaking, distinguished by a more radical activism. The example of Hussein has made the concept of martyrdom more central to Shia than to Sunni traditions.
American soldiers now face a potentially brutal fight against President Saddam's forces, in which they must tiptoe around these most potent holy sites as if they were walking on eggshells, or face alienating the Shias they still hope will yet welcome them as liberators.