The Shia: a force to be reckoned with
Shia Muslims were oppressed by Iraq's Baathist regime for more than 30 years and excluded from the highest ranks of power.
They make up the majority of Iraq's population - accounting for as much as 60% - and their support is seen as vital if any new Iraqi government is to have legitimacy.
They were also the largest group by far, to turn out in the January 2005 elections, after their spiritual leader Ayatollah Ali Sistani ruled that voting was a national duty.
The so-called Shia list, a coalition backed by the ayatollah, was allocated just over half the seats in the transitional National Assembly.
Sunni Arabs, who make up barely 20% of Iraq's population, have dominated the politics of modern Iraq since British rule began in 1921. More recently Saddam Hussein's Baath Party was dominated by Sunni Muslims and he centralised power in his Sunni clan.
Oppressed and neglected
The Shia heartland is in the south-east of the country. It includes Basra and the sacred cities of Najaf and Karbala - home to shrines revered by millions of Shia across the East.
The Shia also make up a sizeable minority of the population in the capital Baghdad, where most live in poverty in sprawling slum areas on the outskirts.
One such area is Sadr City - formally called Saddam City. In Saddam's time the million or so Shia inhabitants of this slum lived under constant surveillance by the authorities. In the late 1990s, this oppression led to unrest that shook the government.
Shia both in Iraq and in exile had acknowledged that they had been waiting for Saddam Hussein's overthrow for decades.
Under his rule, Shia opposition groups were fiercely oppressed and political and religious leaders murdered.
As a result, the opposition tended to look to neighbouring Iran, which is also governed by Shia religious leaders, for support.
In the late 1970s, thousands of Iraqi Shia were expelled to Iran under the pretext of their "Persian connections".
Najaf, 190km south of Baghdad, was once the Shia power centre
Karbala, 80km southwest of Baghdad, replaced Isfahan in Iran as the centre of Shia scholarship
Samarra, 95km north of Baghdad, sacred tombs of Ali al-Hadi and al-Hasan al-Askari, the 10th and 11th Imams, and the site where the 12th Imam, Mohammed al-Mahdi, disappeared
In 1991, after the Gulf War, the first President George Bush encouraged Iraqis to rise up against their leader. The Shia believed this would mean the US would back a rebellion.
But, lacking US support, a massive southern rebellion was brutally suppressed by Saddam Hussein's cousin Ali Hassan al-Majid - otherwise known as Chemical Ali. He was captured by American troops in 2003.
The defeat of the uprising deeply alienated the Shia.
In early Islamic history the Shia were a political faction ("party of Ali") that supported the power of Ali, son-in-law of the Prophet Mohammed and the fourth caliph (temporal and spiritual ruler) of the Muslim community.
Ali was murdered in 661AD and his chief opponent, Muawiya, became caliph. It was Ali's death that led to the great schism between Sunnis and Shias.
Caliph Muawiya was later succeeded by his son Yazid, but Ali's son Hussein refused to accept his legitimacy and fighting between the two resulted.
Hussein and his followers were massacred in battle near Karbala in AD680.
Both Ali and Hussein's death gave rise to the Shia cult of martyrdom and sense of betrayal.
Shia has always been the rigid faith of the poor and oppressed waiting for deliverance. It is seen as a messianic faith which awaits the coming of the "hidden Imam", Allah's messenger who will reverse their fortunes and herald the reign of divine justice.
Today, they make up about 15% of the total worldwide Muslim population.