By David Gritten BBC News website

For more than 1,000 years, Iraq has served as a battleground for many of the events that have defined the schism between Sunni and Shia Muslims.

Sectarian tension has been a catalyst for violence in Iraq

In more recent decades, the political and economic dominance of Iraq's minority Sunni Arabs and their persecution of the country's Shia majority have only served to stoke sectarian tensions.

The US-led invasion in 2003, in which the nominally secular Baath government of Saddam Hussein was overthrown, finally gave Iraq's Shias an opportunity to seek redress and end the imbalance of power.

Though sectarian tension has undoubtedly been a major catalyst of the violence that has plagued Iraq since the invasion, many argue that blaming sectarianism alone overstates the case.

Sunnis and Shias differ in doctrine, ritual, law, theology and religious organisation. It is the largest and oldest division in the history of Islam.

But the origins of the split lie in a dispute over who should have succeeded the Prophet Muhammad as leader of the Muslim community when he died in 632.

One group of Muslims elected Abu Bakr as the next caliph (leader) of the community, but another group believed the prophet's son-in-law, Ali, was the rightful successor.

Shias re-enact the battle near Karbala in which Hussein was killed

Though Ali eventually became the fourth caliph, his legitimacy was disputed and he was murdered in 661.

The Shiat Ali ("Party of Ali") refused to recognise the legitimacy of his chief opponent and successor, Muawiya.

Ali's sons Hassan and Hussein continued to oppose Muawiya and his successor, Yazid, and fighting between the two sides resulted. Hassan was poisoned in 669 and Hussein was killed in battle near Karbala in 680.

Ali, Hassan and Hussein became the first of the 12 imams who Shia Muslims believe are the divinely-appointed leaders of the Muslim community.

The leadership by imams continued until 878, when the 12th Imam, Mohammed al-Mahdi, is said to have disappeared from a cave below a mosque in Samarra.

Not accepting that he died, Shias still await his return more than 1,100 years later. The Hidden Imam's arrival will, they believe, reverse their fortunes and herald the reign of divine justice.

Sunnis, as they became known, reject the principle of leadership by imams, and instead believe in the primacy of the Sunna - what the Prophet Muhammad said, did, agreed to or condemned.

Sunnis are the majority sect in the Muslim world, but Shias today form as much as 60% of Iraq's population, whereas Sunnis make up 37%, split between ethnic Arabs and Kurds.

This demographic dominance has not, however, been translated into economic and political power. Instead, Sunni Arabs have traditionally formed Iraq's elite.

Click here to see where Iraq's ethnic and religious groups live Enlarge Map

The ascendancy of Iraq's Sunni community began under the Sunni Ottoman Turks, whose empire ruled the Middle East for nearly 400 years. Ottoman defeat in World War I did not end Sunni dominance.

In the 1920 Mandate of Iraq, the British worked to check the Shia majority's power by keeping Sunni Arabs in senior positions in government and the armed forces.

The Sunni officers in the army became increasingly politicised and eventually overthrew the British-appointed monarchy in 1958.

The coup by the secular Arab Socialist Baath Party five years later did not redress the inequalities, as the Sunni Arab elite were unaffected.

Increasingly disenfranchised and concerned by the growth of secular parties supported by the government, Shias mobilised around prominent clerics and began to campaign for a return to Islamic principles in government and social justice.

In 1979, the Islamic Revolution in Iran - where Shias constitute 89% of the population - galvanised Shia opposition to the Baath Party and made Saddam Hussein, now president, increasingly fearful of a similar revolution in Iraq.

Who are the Iraqi Shia?

When Shia political activists attempted to assassinate the deputy prime minister in 1980, Saddam responded by executing Ayatollah Mohammed Baqir Sadr, the uncle of radical cleric Moqtada Sadr, the first time so senior a cleric had been killed.

When Iraq declared war on its predominantly Shia neighbour, Iran, Saddam's government intensified its brutal crackdown.

Thousands of Shia were expelled to Iran or imprisoned, tortured and killed. Religious practices were restricted and pilgrimages to holy shrines were curtailed.

In 1991, after the Gulf War, the US President George Bush encouraged Iraqis to rise up against their leader.

Lacking US support, the massive southern rebellion was swiftly and brutally suppressed.

After Saddam was overthrown in 2003, the Sunni Arab supremacy was suddenly wiped out.

The US-led Coalition Provisional Authority's programme of de-Baathification saw large parts of the Sunni elite, which had been nurtured by Saddam, ejected.

They were replaced by Shia leaders who could claim legitimately that they represented the majority of Iraq's population.

Before the bombing, the al-Askari shrine dominated Samarra's skyline

Sunni Arabs resented the appointments and, feeling increasingly marginalised, boycotted the political process and began to support militants opposing the occupation.

Though attacks were initially aimed at foreign troops, Sunni extremist groups, such as Abu Musab al-Zarqawi's al-Qaeda in Iraq, began to advocate targeting the now dominant Shia community.

The view, held by some Sunnis, that Shia Islam is a heterodox sect, fuelled this sectarian killing. In the eyes of some extremists it was an extenuation of ancient hatred and rivalries.

Insurgents have attacked Shia Islam's most important shrines at Karbala, Najaf and Samarra and killed many Shia politicians, clerics, soldiers, police and civilians.

Such attacks have clearly raised sectarian tension to a new level, but they are not the only cause of the violence that has plagued Iraq.

Ethnic conflict and tribalism have contributed to the country's instability in recent decades.

Political groups have also played an important role, with Iraqis subscribing to a broad spectrum of ideologies and affiliations, many of which have nothing to do with religion.

Many Iraqis would argue that their society, in particular in the capital Baghdad, is in fact largely cosmopolitan and that class and social status are of greater significance than religion.