What were the achievements of caliph Abū Bakr, the first caliph of Islam?

Abū Bakr, who was to become the first caliph of Islam, was a close friend of the Prophet Muhammad and one of his earliest converts. One proof of the closeness of their friendship was that the Prophet Muhammad took his daughter Aisha bint Abu Bakr as a second or third wife (sources disagree about the precise timing); Aisha herself became a distinguished scholar, narrating 2210 hadith.

Abū Bakr accompanied Muhammad to Medina in 622, acted as an adviser to Muhammad for many years, and led a pilgrimage to Mecca in 631. In 632, after much debate, he was acclaimed the  khalīfat rasūl Allāh (successor or caliph to the prophet of God.) As he died in 634, his caliphate lasted only two years. The Muslims who supported Abū Bakr and successive caliphs evolved into what are now followers of Sunni Islam; those who opposed him and supported the claims of Ali ibn Abi Talib became what are now the Shiite Muslims. 

Abū Bakr's main achievement during this period was to unify the Islamic world under the central authority of Medina in the “wars of apostasy”, a series of civil wars against other Muslims intended to unify Islam and suppress dissent. His success in these wars led to a strong central government in the Arabian peninsula and the beginning of the expansion of Islam into Syria and Iraq. He also work to collect and preserve the sayings of the Prophet in writing, into a form that became the Koran.