The ancient enmity between Shia and Sunni Muslims exploded in violence again on Saturday in Iraq when a suicide bomb killed at least 50 Shia pilgrims and wounded almost 100 in the southeastern city of Basra, according to Iraqi police and security officials.

A terrorist wearing a police uniform and carrying fake police ID managed to reach a police checkpoint and blew himself up among police and pilgrims, a police official told Reuters.

The pilgrims were celebrating the festival of Arbain, a principal holy day for Shias.

Violence between majority Shia and minority Sunni Muslims has intensified since the departure of U.S. troops in Iraq, raising alarms the country is on the brink of a disastrous sectarian war. Scores of people have been killed in the past few weeks alone.

The conflict has spread to the very highest reaches of Iraqi politics. Last month, President Nouri al-Maliki (a Shiite) ordered the arrest of one of the most prominent Sunni lawmakers in the country, Vice-President Tariq al-Hashemi, on charges of terrorism.

Hashemi is in hiding in the semi-autonomous Kurdish north of Iraq.

Meanwhile, other Sunni politicians have boycotted the cabinet and parliament, accusing Maliki of seeking to marginalize or even eliminate the Sunni presence and influence in government.

But what is behind this endemic hatred and violence between two factions of the same religion?

The split goes all the way back to the death of the Prophet Muhammad in the year 632 and it had to do with succession -- that is, who would be the rightful successor to the Prophet?

Essentially, those who became Sunni believed that the heir to Mohammad should be determined by the community of elder Muslim clerics. To the contrary, those who became Shias (a distinct minority), felt Mohammad’s successor should come from the Prophet's own family, namely Ali, his son-in-law -- since Mohammad had no sons who survived into adulthood.

Soon after, the Sunnis got their way and chose another successor as the first caliph. Ali eventually became the fourth caliph -- but by then it was too late, the cracks had already formed.

In fact, Ali's selection prompted a war leading to his own death in 661 in Kufa, present-day Iraq.

The relationship between the Shia and Sunnis would forever be ruptured.

Moreover, it was Ali's son Hussein who perhaps became the most potent symbol to Shia Muslims.

Hussein launched a war against the reigning caliph -- a bloody battle that led to his own gruesome death. Hussein was decapitated and his severed head was carried to the Sunni caliph in Damascus as a tribute. His (headless) body was left on the battlefield at Karbala (present day Iraq), where it was later buried.

Consequently, Hussein became an eternal martyr for the Shias.

For the past fifteen centuries, Shia Muslims commemorate Hussein's death by self-flagellation in a public ritual called ‘Ashoura.’

Another key difference between Shias and Sunnis has to do with how they address and regard their senior clerics. Shias call their leaders 'Imam' (Ali was the first Imam, Hussein, the third).

The Sunni clerics have no such glorified equivalent.

Gregory Gause, professor of Middle East politics at the University of Vermont, explained to NPR: Some of the Sunnis believe that some of the Shia are actually attributing almost divine qualities to the imams, and this is a great sin, because it is associating human beings with the divinity. And if there is one thing that's central to Islamic teaching, it is the oneness of God.

Yet another key theological dispute between the warring camps has to do with the idea of a Messiah. As with Jews and Christians, Shia Muslims believe that the 12th (and last) Imam -- who lived in the 10th century and vanished, and is known as the Mahdi -- will return to earth at the end of time.

In Iran, the ayatollahs serve as caretakers of the faith until the Mahdi returns.

Sunnis find this belief anathema.

On the other hand, Shias frequently curse the original Caliphs so revered by the Sunnis.

One thing to remember is that Sunnis represent an overwhelming majority of the global Muslim population – perhaps has much as 85 to 90 percent, according to scholarly estimates. Indeed, the only major Muslim country dominated by Shias is Iran, although significant Shia communities exist in Iraq, Lebanon, Saudi Arabia, Syria, Afghanistan, Yemen, Pakistan and India.

Moreover, Yitzhak Nakash, author of ‘The Shias of Iraq,’ estimates that Shias account for some 80 percent of the population in the oil-rich Persian Gulf region (which includes Iran, southern Iraq and eastern Saudi Arabia), thereby providing them with considerable economic leverage.

Iran (ancient Persia) became a Shia kingdom in the 16th-century with the arrival of the Azeris and the establishment of the Safavid dynasty. Iran was thus surrounded by Sunni empires to the east (Mughal India) and to the west (Ottoman Turkey).

The Shia-Sunni conflict is particularly volatile in Iraq. However, some analysts think the ideological differences are being exploited by politicians.

Juan Cole, professor of Middle East History at the University of Michigan, told NPR: “I don't think people are killing each other over... minor differences. I think they're killing each other because these religious ideologies are now being marshaled in a quest for power.”
(Interestingly, Saddam Hussein was Sunni, although he included many Shias among his inner circle)

Bahrain presents a particularly interesting stage for the Shia-Sunni saga. The tiny kingdom's population has a Shia majority, but is ruled by a Sunni dynasty. When anti-regime protests broke out last spring, the Bahraini king recruited military assistance from neighboring Saudi Arabia (a Sunni power) to help stamp out the rebellion.

Meanwhile, Shia-dominated Iran bitterly criticized the Saudi intervention in Bahrain. In turn, the Saudis openly accused Teheran of fomenting the unrest in Bahrain.

Saudi Arabia has its own problems with its restless Shia minority who dominate the oil-rich eastern corridor of the kingdom.

Saudi officials have responded to sporadic demonstrations with brutal, decisive force -- and, again, have explicitly blamed Iran for stirring up the trouble.