The caliph was often known as Amir al-Mu'minin, or "Commander of the Believers". Muhammad established his capital in Medina, and after he died it remained the capital for the Rashidun period. The first caliph to be called Amir al-Mu'minin was Abu Bakr Siddique, followed by Umar ibn al-Khattāb, the second of the Four Rightly Guided Caliphs. Uthman ibn Affan and Ali ibn Abi Talib also were called by the same title.
After the first four caliphs, the Caliphate was claimed by dynasties such as the Umayyads, the Abbasids, and the Ottomans, and for relatively short periods by other, competing dynasties in al-Andalus, North Africa, and Egypt. Mustafa Kemal Atatürk officially abolished the last Caliphate, the Ottoman Empire, and founded the Republic of Turkey, in 1924. The Kings of Morocco still label themselves with the title Amir al-Mu'minin for the Moroccans, but lay no claim to the Caliphate.
Abu Bakr, the first successor of Muhammad, nominated Umar as his successor on his deathbed, and there was consensus in the Muslim community to his choice. Umar Ibn Khattab, the second caliph, was killed by a servant. His successor, Uthman Ibn Affan, was elected by a council of electors (Majlis), but was soon perceived by some to be ruling as a "king" rather than an elected leader. Uthman was killed by members of a disaffected group. Ali then took control but was not universally accepted as caliph by the governors of Egypt, and later by some of his own guard. He faced two major rebellions and was assassinated after a tumultuous rule of only five years.
Muawiyah, a relative of Uthman and governor (Wali) of Syria, became one of Ali's challengers and after Ali's death managed to overcome the other claimants to the Caliphate. Muawiyah transformed the caliphate into a hereditary office, thus founding the Umayyad dynasty. Under the Umayyads, the Caliphate grew rapidly in territory. Islamic rule expanded westward across North Africa and into Hispania and eastward through Persia and ultimately to the ancient lands of Indus Valley, in modern day Pakistan. This made it one of the largest unitary states in history and one of the few states to ever extend direct rule over three continents (Africa, Europe, and Asia).
Source: Wikipedia English, 17 February 2011