Abu Bakr
Muhammad's closest companion and the first caliph, whose two-year rule held the Muslim community together and whose selection opened the Sunni-Shia split.
When Muhammad died in Medina in 632 he left no son, no named successor that everyone agreed on, and a political community that had existed for barely a decade. Within forty-eight hours it had a leader, and within two years that leader had held it together, crushed a general revolt, and launched the armies that would take Persia and half the Roman East. His name was Abu Bakr, and both of the great facts about him, that he saved the community and that his selection divided it permanently, are still live issues fourteen centuries later.
He was a Meccan cloth merchant, roughly Muhammad's age, and by most accounts the first adult male outside the Prophet's household to accept his message. He spent his fortune buying and freeing enslaved converts. He was the sole companion on the hijra, the migration to Medina in 622, hiding with Muhammad in the cave outside Mecca while the search parties passed. His daughter Aisha became the Prophet's wife. In Muhammad's final illness it was Abu Bakr he sent to lead the communal prayers, a detail his supporters would later treat as decisive.
Saqifa
The succession was settled in a rush, in a roofed meeting place called Saqifa Bani Saida, where the Medinan clans had gathered to choose one of their own. Abu Bakr and Umar went to argue that leadership had to stay with the Meccan emigrants, and the assembly acclaimed Abu Bakr as khalifat rasul Allah, successor to the Messenger of God. It was the origin of the office of caliph.
It was also, in the eyes of a minority, a coup. Muhammad's cousin and son-in-law Ali ibn Abi Talib was occupied with the Prophet's burial and was not there. Those who believed Muhammad had designated Ali as his heir, above all at a place called Ghadir Khumm, saw the meeting as a decision made by the wrong people while the right man was absent. That grievance is the seed of the Sunni-Shia split. Sunnis regard Abu Bakr as the first of four Rightly Guided Caliphs; Shia regard him as the first usurper of Ali's right.
The Wars of Apostasy
The practical crisis was immediate. Many Arabian tribes had sworn allegiance to Muhammad personally and considered the deal expired with him; some stopped paying zakat, the alms tax, and several backed rival prophets of their own. Abu Bakr refused every compromise, holding that a tribe that withheld the tax had left the faith. The resulting Ridda wars, the wars of apostasy, ran through 632 and into 633 and were won largely by the commander Khalid ibn al-Walid, whose bloodiest victory came at Yamama against the prophet Musaylima.
Two consequences followed. Arabia was unified by force under a single religious and political authority, for the first time in its history. And so many men who had memorized the revelations died at Yamama that Abu Bakr was persuaded to have them collected in written form, the first step toward the standardized Quran.
He then turned the newly unified tribal armies outward, into Sasanian Iraq and Byzantine Syria, which conveniently gave restless warriors somewhere to fight that was not each other. He died in August 634 after roughly two years in power, having named Umar as his successor directly, and was buried beside Muhammad.
His short rule set two precedents that outlasted every caliphate. The first: the community, not a bloodline, chooses its leader, the Sunni position. The second, and the one his opponents drew: it chose wrong the very first time, and every generation since has had reason to argue about it.