Larbi Sadiki is a lecturer on Middle East politics at Exeter University in Britain. He says the schism between Sunni and Shi'a reaches back to quarrels over who should succeed Muhammad as caliph, the spiritual and temporal leader of Muslims.
"Historically, it really has a lot to do, essentially, with the question of [succession] -- whether the fourth caliph should have been, [whether] the successor of the Prophet [Muhammad] should have been Ali [ibn Abi Talib] or should have been somebody else. There's contention and controversy over this issue, and that's really the main issue," Sadiki said.
In the end, Ali -- who was a cousin and son-in-law of Muhammad -- became the fourth caliph in 656. He immediately faced a challenge by other pretenders to the caliphate. In 661, Ali was assassinated and was buried in Al-Najaf, a city in Iraq that is one of the most sacred places for the Shi'a.
After his death, Ali's son al-Hussein was murdered with his fighters at Karbala in 680. His death is the central event in Shi'ism.
Sunnis respect Ali as the fourth caliph but do not regard him with as much reverence as the Shi'a.
Consequently, Shi'a religious doctrine is significantly different from that of Sunnis. The main difference lies in the institution of the imamate, which is not present in the Sunni branch. In Sunni mosques, an imam is the leader of prayer and, in his absence, any male may serve as imam.
The Shi'a consider the imam to be a more exalted position.
"The imamate is really an institution which is specific to Shi'a Islam -- no doubt about that,” Sadiki says. “I guess that the crux of it is that the imam -- and I guess especially after the Iranian revolution -- it's become really pragmatic. It's been given a pragmatic meaning."
The Shi'a believe there can be only one imam at a time. He is credited with possessing supernatural knowledge and authority almost equal to the Prophet. For the majority of Shi'a -- who are called Twelve-Imam Shi'a -- the imam is an intermediary between man and God. In addition to his spiritual authority, the imam has an absolute right to civil authority.
The 12th and the last Shi'a imam, Muhammad al-Mahdi-l-Muntazar, disappeared in the year 873. It is believed that he -- as the mahdi, or truly guided one -- will return at the end of time and bring justice to the world. He is known as the Hidden Imam. Shi'a believe the Hidden Imam illuminates men and intercedes with God on their behalf. This belief is strongly rejected by Sunnis as being Messianic and incompatible with Islam.
Thus, the Shi'a still accept "independent reasoning" in analyzing religious issues, but insist that every believer be guided by one of a very few living senior clerics, who are considered "sources of emulation." The institution of the imamate allows Shi'a clergy to influence not only spiritual matters but also politics. Shi'a clerics believe they have the inherited right to influence social matters.
This doctrine explains the authority attributed by Shi'a in Iraq to the rulings of the country's top Shi'a cleric, Grand Ayatollah Ali al-Sistani.
"There is actually so much space for people like [al-Sistani] to partake in political affairs because of the silence of the religious texts on so many mundane matters, or matters having to do with the horizontal dimension of life -- that is, like politics, economics, and so on," Sadiki said.
Sadiki says this institution makes the Shi'a closer to the concept of theocracy.
In contrast, Sunnis depend on a long-developed corpus of law, in which only precedent or analogy can be used and therefore rulings by leading contemporary clerics are not considered decisive.
Although Sunnis and Shi'a have some different attitudes to life and human behavior, analysts say both branches accept the basic truths of Islam and should not be considered competing forces. Nevertheless, tensions exist.
Nations with Sunni majorities include Egypt, Saudi Arabia, and most other Arab nations, as well as Pakistan, Turkey, and Afghanistan. Most Palestinian Muslims are Sunnis.
Shi'a Islam is the official state religion in Iran, where it enjoys an overwhelming majority. Shi'a make up about 65 percent of the population in Iraq but were harshly repressed under the Saddam Hussein regime. Lebanon and Bahrain also have large Shi'a communities.
Mustafa Alani is an associate fellow in the Middle Eastern program at the Royal United Services Institute for Defense Studies in London. He says the Shi'a experience difficulties in countries where they are in the minority.
"They might have a problem in a country like Saudi Arabia, where Shi'a is a small minority. They might have a problem there. There's a problem in Pakistan, for example, for the last ten years. But in general, I don't think there is a state of conflict between Shi'as and Sunnis, and there was no civil war in the history of the Middle East which was based on this division -- Shi'a or Sunni. The civil war was always on ethnic [lines]," Alani said.
But Sadiki says tensions exist in almost every Muslim country where the two communities co-exist. He says some Sunni fundamentalist groups often use the term "infidels" when referring to the Shi'a.
He also says ordinary Muslims -- both Sunni and Shi'a -- place great importance on their religious differences.
"Definitely, when you talk [about] 'folk Islam' -- the Islam that is practiced by the people -- it's definitely more rigid. It's more fanatic in the sense that people really cling to rigidity. They cling to divisiveness and lots of people in the street sort of see themselves [as] if they were Shi'as or Sunnis -- you know, to be different from the other side," Sadiki said.