The term "Caliphate" refers to the political-religious state comprising the Muslim community and the lands and peoples under its dominion in the centuries following the death (AD 632) of the Prophet Muḥammad. Ruled by a caliph (Arabic khalîfah, “successor”), who held temporal (and sometimes a degree of spiritual) authority, the empire of the Caliphate grew rapidly through conquest during its first two centuries to include most of Southwest Asia, North Africa, and Spain. Dynastic struggles later brought about the Caliphate’s decline, and it ceased to exist with the Mongol destruction of Baghdad in 1258.
The urgent need for a successor to Muḥammad as political leader of the Muslim community was met by a group of Muslim elders in Medina who designated Abû Bakr, the Prophet’s father-in-law, as caliph. Several precedents were set in the selection of Abû Bakr, including that of choosing as caliph a member of the Quraysh tribe. The first four caliphs—Abû Bakr, ʿUmar I, ʿUthmân, and ʿAlî—whose reigns constituted what later generations of Muslims would often remember as a golden age of pure Islâm, largely established the administrative and judicial organization of the Muslim community and forwarded the policy begun by Muḥammad of expanding the Islâmic religion into new territories. During the 630s, Syria, Jordan, Palestine, and Iraq were conquered; Egypt was taken from Byzantine control in 645; and frequent raids were launched into North Africa, Armenia, and Persia.
The assassination of ʿUthmân, and the ineffectual caliphate of ʿAlî that followed, sparked the first sectarian split in the Muslim community. By 661 ʿAlî’s rival Muʿâwiyah I, a fellow member of ʿUthmân’s Umayyad clan, had wrested away the Caliphate, and his rule established the Umayyad caliphate that lasted until 750. Despite the largely successful reign of Muʿâwiyah, tribal and sectarian disputes erupted after his death. There were three caliphs between 680 and 685, and only by nearly 20 years of military campaigning did the next one, ʿAbd al-Malik, succeed in reestablishing the authority of the Umayyad capital of Damascus. ʿAbd al-Malik is also remembered for building the Dome of the Rock in Jerusalem. Under his son al-Walîd (705–715), Muslim forces took permanent possession of North Africa, converted the native Berbers to Islâm, and overran most of the Iberian Peninsula as the Visigothic kingdom there collapsed. Progress was also made in the east with settlement in the Indus River valley. Umayyad power had never been firmly seated, however, and the Caliphate disintegrated rapidly after the long reign of Hishâm (724–743). A serious rebellion broke out against the Umayyads in 747, and in 750 the last Umayyad caliph, Marwân II, was defeated in the Battle of Great Zab by the followers of the ʿAbbâsid family.
The ʿAbbâsids, descendants of an uncle of Muḥammad, owed the success of their revolt in large part to their appeal to various pietistic, extremist, or merely disgruntled groups, and in particular to the aid of the Shîʿites, a major dissident party that held that the Caliphate belonged by right to the descendants of ʿAlî. That the ʿAbbâsids disappointed the expectations of the Shîʿites by taking the Caliphate for themselves left the Shîʿites to evolve into a sect, permanently hostile to the orthodox Sunnite majority, that would periodically threaten the established government by revolt. The first ʿAbbâsid caliph, as-Saffâḥ (749–754), ordered the elimination of the entire Umayyad clan; the only Umayyad of note who escaped was ʿAbd ar-Raḥman, who made his way to Spain and established an Umayyad dynasty that lasted until 1031.
The period 786–861, and especially the caliphates of Hârûn (786–809) and al-Maʾmûn (813–833), is regarded to have been the height of ʿAbbâsid rule. The eastward orientation of the dynasty was demonstrated by al-Manṣûr’s removal of the capital to Baghdad in 762–763 and by the later caliphs’ policy of marrying non-Arabs and recruiting Turks, Slavs, and other non-Arabs as palace guards. Under al-Maʾmûn, the intellectual and artistic heritage of Iran (Persia) was cultivated, and Persian administrators assumed important posts in the Caliphate’s administration. After 861, anarchy and rebellion shook the empire. Tunisia and eastern Iran came under the control of hereditary governors who made token acknowledgment of Baghdad’s suzerainty. Other provinces became less reliable sources of revenue. Shîʿite and similar groups, including the Qarmaṭians in Syria and the Fâṭimids in North Africa, challenged ʿAbbâsid rule on religious as well as political grounds.
ʿAbbâsid power ended in 945, when the Bûyids, a family of rough tribesmen from northwestern Iran, took Baghdad under their rule. They retained the ʿAbbâsid caliphs as figureheads. The Sâmânid dynasty that arose in Khorâsân and Transoxania and the Ghaznavids in Central Asia and the Ganges River basin similarly acknowledged the ʿAbbâsid caliphs as spiritual leaders of Sunnî Islâm. On the other hand, the Fâṭimids proclaimed a new caliphate in 920 in their capital of al-Mahdîyah in Tunisia and castigated the ʿAbbâsids as usurpers; the Umayyad ruler in Spain, ʿabd ar-Raḥmân III, adopted the title of caliph in 928 in opposition to both the ʿAbbâsids and the Fâṭimids.
Nominal ʿAbbâsid authority was restored to Egypt by Saladin in 1171. By that time, the ʿAbbâsids had begun to regain some semblance of their former power, as the Seljuq dynasty of sultans in Baghdad, which had replaced the Bûyids in 1055, itself began to decay. The caliph an-Nâṣir (1180–1225) achieved a certain success in dealing diplomatically with various threats from the East, but al-Mustaʿṣim (1242–58) had no such success and was murdered in the Mongol sack of Baghdad that ended the ʿAbbâsid line in that city. A scion of the family was invited a few years later to establish a puppet caliphate in Cairo that lasted until 1517, but it exercised no power whatever.
In modern times, many Islamist groups and their leaders seek to restore the Caliphate and, by extension, Islam's former glory. Among the organizations promoting this objective are the Muslim Brotherhood, Hizb-ut-Tahrir, Al Muhajiroun, As-Sabiqun, and al Qaeda.
Most of this introduction is from Britannica.com's entry for "Caliphate."