A sacrilegious blow that goes to the heart of Islam's historic schism

It is as if bombs had been placed in the Church of the Holy Sepulchre in Jerusalem on Good Friday - with a few more for good measure at the Vatican and in Canterbury Cathedral. Yet even that comparison fails to convey the outrageous and sacrilegious impact of the blasts that killed scores of Iraqi Shia worshippers yesterday as they celebrated Ashoura, their most holy ritual.

The festival commemorates the martyrdom of Imam Hussein, the grandson of the Prophet Mohammed. It lies at the heart of Islam's historic rift between the Sunni and the Shia - and is seen by Shias as the greatest suffering and redemptive act in history, much in the way that Christians view the death of Christ.

The great schism in Islam began almost immediately upon the death of Mohammed AD632 with a dispute over who should succeed him. The split eventually led to the murder of one contender, the Prophet's son-in-law Ali. Nineteen years later Ali's son, Hussein, set out with just 72 supporters "to deliver Iraq from the pretender", as Shia historians put it.

They took on thousands of soldiers from the other side. They were killed in AD680 at Karbala by the army of the Sunni caliph Yazid. On the 10th day, now known as Ashoura, the defenders fell one by one until only Hussein remained.

Wounded and dying, he entered his tent and took his infant child in his arms, but the enemy killed him with an arrow as he lifted his hands to the heavens. They then cut off his head and trampled on his body. This bloody death has since assumed cosmic proportions. Shias believe that Imam Hussein, by deliberately taking on thousands of enemy soldiers, knowingly sacrificed himself for justice, freedom and peace on behalf of all humanity.

Over the centuries the Shia have developed graphic forms of penance to atone for the martyrdom of the great leader. For 10 days they fast, wear black, attend vigils and conduct processions to express their grief at the death of the Imam, his family and his followers. These expressions of sorrow are more concentrated and extreme than anything else in the Shia tradition.

The faithful beat their chests with the palms of their hands and mortify themselves with stones. The more zealous flay their backs with chains. Participation in the rituals is believed to be an aid to salvation.

But there is something more. Imam Hussein's battle against the forces of darkness is believed to have transcended the particularity of time and place. The fight is against the injustice, tyranny or oppression of the present day.

Thus the mixing of Ashoura chants with political slogans is a Shia tradition. And Muharram, the month of which Ashoura is the 10th day, is charged with greater emotion than anything else in the Shia calendar.

The Shia clerics who led the Iranian Revolution were careful to frame the revolt against the decadent Western regime of the Shah in an Ashoura/ Hussein paradigm. One of the great slogans of the Iranian Revolution was: "Everyday is Ashoura; every place is Karbala." Ayatollah Khomeini issued a proclamation describing the month as one of "epic heroism and self-sacrifice".

Small wonder that in his time Saddam Hussein tried hard to suppress Ashoura celebrations, imprisoning those seen to strike their breast in public. The Baathist security forces and the army used to surround Karbala for two months to keep worshippers from practising the traditional rituals.

Checkpoints were set up on the roads leading to the holy city or the shrine at Najaf, where Hussein's father, Ali, is buried. Those who tried to pass the checkpoints were killed or arrested. Saddam even banned books that mentioned Imam Hussein's name or the story of the battle of Karbala.

When Saddam's regime was toppled, hundreds of thousands of Shia spontaneously made for Karbala, but this year was to have been the first in which Ashoura was to be celebrated openly by the Shia population. Whoever planned yesterday's bombs could not have chosen a more potent place or time.