After the death of Muhammad in 632, a group of Muslims, who would come to be known as the Sunnis, believed that Muhammad's successor as caliph of the Islamic community should be Abu Bakr, whereas a second group of Muslims, who would come to be known as the Shias, believed that his successor should have been Ali. This dispute spread across various parts of the Muslim world, which eventually led to the Battle of Jamal and Battle of Siffin. Sectarianism based on this historic dispute intensified greatly after the Battle of Karbala, in which Husayn ibn Ali and some of his close partisans, including members and children of the household of prophet (Ahl al bayt), were killed by the ruling Umayyad Caliph Yazid I, and the outcry for revenge divided the early Islamic community, albeit disproportionately, into two groups, the Sunni and the Shia. This is known today as the Islamic schism.
The present demographic breakdown between the two denominations is difficult to assess and varies by source, with most approximations stating that roughly 90% of the world's Muslims are Sunni and 10% are Shia; with about 85% of Shias belonging to the Twelver tradition, and the rest divided between other small groups. Sunnis are a majority in almost all Muslim communities around the world. Shia make up the majority of the citizen population in Iran, Iraq, Bahrain, and Azerbaijan, as well as being a minority in Pakistan, Lebanon, Saudi Arabia, Syria, Yemen, Nigeria, Afghanistan, Chad, Turkey and Kuwait.
Today there are differences in religious practice, traditions, and customs, often related to jurisprudence. Although all Muslim groups consider the Quran to be divine, Sunni and Shia have different opinions on hadith.
In recent years, the Sunni–Shia divide has been increasingly marked by conflict, particularly the Iran–Saudi Arabia proxy conflict. Sectarian violence persists to this day from Pakistan to Yemen and is a major element of friction throughout the Middle East and South Asia. Tensions between communities have intensified during power struggles, such as the Bahraini uprising, the Iraqi Civil War, the Syrian Civil War, as well as the War in Iraq (2013–2017), during which the self-styled Islamic State of Iraq and Syria launched a persecution of Shia.
Numbers
Sunni Muslims are the vast majority of Muslims in most Muslim communities in Central Asia (including China), Europe (including Russia and the Balkans), South Asia, Southeast Asia, Africa, Arab World, Turkey and among Muslims in the United States.
Shia Muslims make up approximately 10% of the Muslim population. In Shi'i Islam itself, about 85% are Twelver, and in Twelver Shia the overwhelming majority are of the Usuli school. In Iran -- a Shia country since the 16th century -- Shia make up the majority (around 90%). They are also a majority in Azerbaijan (around 65%), Iraq (around 55%) and Bahrain (around 60% of the citizens, excluding expatriates). Shia communities are also found in Yemen where a large minority of the population are Shia (mostly of the Zaidi sect), according to the UNHCR. Sources put the numbers of Shia in Yemen at 25–30%. About 10% of Turkey's population belong to the Alevi sect of Shi'i Islam. The Shia constitute around 20% of Kuwait, 45% of the Muslim population in Lebanon, 10% of Saudi Arabia, 12% of Syria (mostly of the Alawite sect), and 10% of Pakistan. Around 10% of Afghanistan, less than 5% of the Muslims in Nigeria, and around 5% of population of Tajikistan are Shia. India has as many Shia ("potentially") as there are in Iraq.
Scholar Vali Nasr argues that numbers/percentages of how many Sunni and Shia there are, are not exact because "in much of the Middle East it is not convenient" to have exact numbers, "for ruling regimes in particular".
Differences in beliefs and practices
Successors of Muhammad
Mahdi
The Mahdi is the prophesied redeemer of Islam. While Shia and Sunnis differ on the nature of the Mahdi, many members of both groups believe that the Mahdi will appear at the end of the world to bring about a perfect and just Islamic society.
In Shia Islam, "the Mahdi symbol has developed into a powerful and central religious idea." Twelvers believe the Mahdi will be Muhammad al-Mahdi, the twelfth Imam returned from the Occultation, where he has been hidden by Allah since 874. Mainstream Sunnis beliefs are somewhat different. The Mahdi forms an important component of Sunni eschatology, his appearance being considered the last of the minor signs of the Day of Judgment before its major signs. They believe the Mahdi will be a descendant of Muhammad named Muhammad, and will revive the faith.