The main difference between Sunnis and Shias lies in their interpretation of the rightful succession of leadership after the death of the prophet Muhammad. The declaration of faith to which all Muslims assent is this: There is no God but Allah, whose prophet is Muhammad. However, the Shiites add an extra phrase at the end: and Ali is the friend of God. Because the Shiites passionately attest to Ali being the successor to Muhammad, much feuding and division have been caused in the world of Islam, not unlike the feuding between Protestants and Roman Catholics in Europe during the Reformation. However, the schism that sets up the major sects of Islam is not due to doctrinal issues, as between Protestants and Catholics, but is grounded in the identity of the “true successor” to Muhammad.
Among the close disciples of Muhammad was Ali, his son-in-law, who was most familiar with his teachings. However, when Muhammad died in A.D. 632, the followers bypassed Ali, whom the Shiites claim as the rightful successor to Muhammad. Instead, a cousin of Muhammad’s third successor, Uthman (A.D. 644-656), called Mu’awiya Umayyad, declared himself caliph.
When he died in A.D. 680, his son Yazid usurped the caliphate instead of Ali’s youngest son, Hussein. The feud between rightful successors or caliphs was fought at the battle of Karbala. Hussein was slain, but his sole son, Ali, survived and continued the line of succession. Yazid, however, gave rise to the Ummayad line of succession, from which modern-day Sunnism arose.
As for their beliefs, both Sunni and Shia Muslims agree on the five pillars of Islam. While the Sunnites honor Ali, they do not venerate their imams as having the gift of divine intercession. Shiites do venerate their imams, believing they are endowed with infallibility in their interpretation of the Qur’an. In many ways, this mirrors the way the Pope is venerated in Rome. Sunnites conduct community prayers and believe they can have a direct relationship with God. Both Shiite Muslims and Sunni Muslims are involved in terrorism. Shiite groups include Hizbollah in Lebanon and the Iranian Revolutionary Guard Corps/Quds Force. Sunni groups include al-Qaeda, ISIS/ISIL, the Taliban in Afghanistan, and Boko-Haram.
In terms of actual practice, the Sunni Muslims pray five times a day: the fajr, the zohr, the asar, the maghrib and finally the isha (“darkness”). Shia Muslims only pray three times—morning, lunchtime and sunset. Another important difference between the two sects is that Shia Muslims permit fixed-term temporary marriage, known as muttah. Muttah was originally permitted at the time of the Prophet and is now being promoted in Iran by an unlikely alliance of conservative clerics and feminists, the latter group seeking to downplay the obsession with female virginity which is prevalent in both forms of Islam, pointing out that only one of the Prophet's thirteen wives was a virgin when he married her.
Iran is overwhelmingly Shia - 89 percent. Shia Muslims also form a majority of the population of Yemen, Azerbaijan, Bahrain and 60 percent of the population of Iraq. There are also sizeable Shia communities along the east coast of Saudi Arabia and in Lebanon. The well-known guerrilla organization Hezbollah, which forced the Israelis out of southern Lebanon in 2000, is Shia. Worldwide, Shias constitute 10 to 15 percent of the overall Muslim population, but they make up the majority of the radical, violent element of Islam.