Abu Bakr in the context of "Early Muslims"

Sunni Islam is the largest branch of Islam and the largest religious denomination in the world. It holds that Muhammad did not appoint any successor and that his closest companion Abu Bakr (r. 632–634) rightfully succeeded him as the caliph of the Muslim community, being appointed at the meeting of Saqifa. This contrasts with the Shia view, which holds that Muhammad appointed Ali ibn Abi Talib (r. 656–661) as his successor. Nevertheless, Sunnis revere Ali, along with Abu Bakr, Umar (r. 634–644) and Uthman (r. 644–656) as 'rightly-guided caliphs'.

The term Sunni means those who observe the sunna, the practices of Muhammad. The Quran, together with hadith (especially the Six Books) and ijma (scholarly consensus), form the basis of all traditional jurisprudence within Sunni Islam. Sharia legal rulings are derived from these basic sources, in conjunction with consideration of public welfare and juristic discretion, using the principles of jurisprudence developed by the four legal schools: Hanafi, Hanbali, Maliki and Shafi'i.

The Rashidun Caliphate (Arabic: ٱلْخِلَافَةُ ٱلرَّاشِدَةُ, romanized: al-Khilāfah ar-Rāšidah) is the early Islamic polity led by the first four successive caliphs (lit. "successors") Abu Bakr, Umar, Uthman and Ali, collectively the Rashidun, or "Rightly Guided" caliphs. These early caliphs led the Muslim community from the death of the Islamic prophet Muhammad in 632 CE to the foundation of the Umayyad Caliphate in 661 CE.

The title Rashidun stems from the Sunni Islamic belief that the caliphs were "rightly guided". Endowed with superior piety and wisdom, their era is regarded in Sunni Islam as a "golden age", second only to the lifetime of the Prophet Muhammad in sanctity and in providing moral and religious guidance. Sunni Muslims consider the "rightly guided" reign of the first four caliphs as a model to be followed and emulated from a religious point of view. The term Rashidun is not used by Shia Muslims, who make up 10–15% of the global Muslim population, as they reject the rule of the first three caliphs as illegitimate.

The issue of succession following the death of the Islamic prophet Muhammad is the central issue in the schisms that divided the early Muslim community in the first century of Islamic history into numerous schools and branches. The two most prominent branches that emerged from these divisions are Sunni and Shia as well as Ibadi branches of Islam. Sunni Islam and Ibadi Islam asserts that Abu Bakr rightfully succeeded Muhammad through a process of election. In contrast, Shia Islam maintains that Ali ibn Abi Talib was Muhammad's designated successor.

These differing viewpoints on succession stem from varying interpretations of early Islamic history and the hadiths, which are the recorded sayings of Muhammad. Sunni Muslims contend that Muhammad did not explicitly appoint a successor, leaving the choice of leadership to the Muslim community. They recognize the legitimacy of Abu Bakr's rule, who was elected at Saqifah, as well as that of his successors, collectively known as the Rashidun caliphs.

The Muslim conquest of the Levant (Arabic: فَتْحُ الشَّام, romanized: Fatḥ al-šām; lit. 'Conquest of Syria'), or Arab conquest of Syria, was a 634–638 CE conquest of Byzantine Syria by the Rashidun Caliphate.

A part of the wider Arab–Byzantine wars, the Levant was brought under Arab Muslim rule and developed into the provincial region of Bilad al-Sham. Clashes between the Arabs and Byzantines on the southern Levantine borders of the Byzantine Empire had occurred during the lifetime of Muhammad, with the Battle of Muʿtah in 629 CE. However, the actual conquest did not begin until 634, two years after Muhammad's death. It was led by the first two Rashidun caliphs who succeeded Muhammad: Abu Bakr and Umar ibn al-Khattab. During this time, Khalid ibn al-Walid was the most important leader of the Rashidun army.

The Saqifa (Arabic: سَقِيفَة, romanized: Saqīfah) of the Banu Sa'ida clan refers to the location of an event in early Islam where some of the companions of the Islamic prophet Muhammad pledged their allegiance to Abu Bakr as the first caliph and successor to Muhammad shortly after his death in 11 AH (632 CE). The Saqifa meeting is among the most controversial events in early Islam, due to the exclusion of a large number of Muhammad's companions, including his immediate family and notably Ali, his cousin and son-in-law. The conflicts that arose soon after Muhammad's death are considered to be the main cause of the current division among Muslims. Those who accepted Abu Bakr's caliphate were later labeled Sunnis, while the supporters of Ali's right to caliphate were later labeled Shia.

Shia Islam is the second-largest branch of Islam. It holds that Muhammad designated Ali ibn Abi Talib (r. 656–661) as both his political successor (caliph) and as the spiritual leader of the Muslim community (imam). However, his right is understood to have been usurped by a number of Muhammad's companions at the meeting of Saqifa, during which they appointed Abu Bakr (r. 632–634) as caliph instead. As such, Sunni Muslims believe Abu Bakr, Umar (r. 634–644), Uthman (r. 644–656) and Ali to be 'rightly-guided caliphs', whereas Shia Muslims regard only Ali as the legitimate successor.

Shia Muslims believe that the imamate continued through Ali's sons, Hasan and Husayn, after which various Shia branches developed and recognized different imams. They revere the ahl al-bayt, the family of Muhammad, maintaining that they possess divine knowledge. Shia holy sites include the shrine of Ali in Najaf, the shrine of Husayn in Karbala, and other mausoleums of the ahl al-bayt. Later events, such as Husayn's martyrdom in the Battle of Karbala (680 CE), further influenced the development of Shia Islam, contributing to the formation of a distinct religious sect with its own rituals and shared collective memory.

Ali ibn Abi Talib (Arabic: عَلِيُّ بْن أَبِي طَالِب, romanized: ʿAlī ibn Abī Ṭālib) (c. 600 – 661 CE) was the fourth Rashidun caliph who ruled from 656 CE until his assassination in 661, as well as the first Shia Imam. He was the cousin and son-in-law of the Islamic prophet Muhammad. Born to Abu Talib ibn Abd al-Muttalib and Fatima bint Asad, Ali was raised by his elder cousin Muhammad and was among the first to accept his teachings.

Ali played a pivotal role in the early years of Islam when Muslims were severely persecuted in Mecca. After the immigration (hijra) to Medina in 622, Muhammad gave his daughter Fatima to Ali in marriage and swore a pact of brotherhood with him. Ali served as Muhammad's secretary and deputy in this period, and was the flag bearer of his army. Numerous sayings of Muhammad praise Ali, the most controversial of which was uttered in 632 at the Ghadir Khumm, "Whoever I am his mawla, then Ali is his mawla." The interpretation of the polysemous Arabic word mawla is disputed: For Shia Muslims, Muhammad thus invested Ali with his religious and political authority, while Sunni Muslims view this as a mere statement of friendship and rapport. When Muhammad died in the same year, a group of Muslims met in the absence of Ali and appointed Abu Bakr (r. 632–634) as their leader. Ali later relinquished his claims to leadership and resigned from public life during the reigns of Abu Bakr and his successor, Umar (r. 634–644). Even though his advice was occasionally sought, the conflicts between Ali and the first two caliphs are epitomized by his refusal to follow their practices. This refusal cost Ali the caliphate to the benefit of Uthman (r. 644–656), who was thus appointed to succeed Umar by the electoral council. Ali was also highly critical of Uthman, who was widely accused of nepotism and corruption. Yet Ali also repeatedly mediated between the caliph and the provincial dissidents angered by his policies.