In Islam, the ruler acting in Muhammad's place after his death, as accepted by the Sunni branch. The institution of the Caliph is called Caliphate.
When Muhammad died in 632, the Muslim community faced a problem on how their community should be governed, and how leaders should be appointed. There were conflicting stories on what Muhammad had said. The tensions developed already with the first appointment of a leader, a person acting in the place of the Messenger, a khalifa ar-rasūl, a few months after Muhammad's death. Khalifa can be translated with 'successor, viceregent', but it is a term that is seldom used for anything other than the leader of the entire Muslim community. But when it is used, as with the honorary titling of good Muslims, it always refers to its standard meaning.
Through history, there have been parallell Caliphs, but none had as much symbolic power and influence as the one that followed the line of Caliphs from Abu Bakr, who was the first. This line of Caliphs had a continuing residence in Damascus from 661 to 750 and Baghdad and Samarra up until 1258. After 1258 and until 1924 there have been several Caliphs, but all of these have had only limited influence. They have represented no continuation of the Caliphs of Baghdad, and in more than one case, these caliphhoods have been motivated by political motives rather than religious ones. The Muslim world has never agreed upon uniting behind any one of these.
The Caliph carried other titles which were less modest, as they were not relative to Muhammad, but to the Muslim community. As amīr al-mu'minīn, he was responsible for the Muslim armies. As imām, he was the head of public worship, and gave khutbas.
The last Caliph was removed by the Mongols when they conquered Baghdad. After this there have been several rulers establishing their own Caliph, but the Caliphate never gained any of its former power or importance.
There are four periods of the Caliphate of Islam:
Rashidun is the name used for the four first Caliphs, from 632 to 661 and indicates that these were the just and admirable leaders of the Muslim community.
This period was marked by a long line of conquests by the Arabs as well as endeavours to turn the leaflets of the revelations that had been given to Muhammad into a book, the Koran. Inside the Muslim realms peace prevailed until the death of Uthman in 656. Because he was murdered, the Muslims could not agree upon exactly who was responsible. At this time, the caliphate of Ali, developed the two schisms that have impregnated Islam ever since. First there was a break between the majority and a group that came to be called Kharijis (surviving as Ibidis), and later a schism occurred between the group now known as Shi'is and the Sunnis.
The Umayyads got their power through military action, a fact that influenced their religious legitimacy strongly through the 90 years during which they held power.
Most Muslims regard the Umayyads as less admirable than both the Rashiduns and the later Abbasids. Even if the Shi'is did not accept the rule of the Umayyad Caliphs, this group was at the time too weak to represent much of a threat to the ruling group.
The Abbasids were to a large extent Shi'is (the lines of division in use today were not as clear in those early days) and the defeat of the Umayyads was strongly motivated by Ali's claim to the leading position in the Muslim world. The Abbasid Caliph involved himself strongly in the religious life of the community. The distance between ruler and people became greater, and the court of the Caliph was one of increasing splendor.
The 9th century initiated the decline of the real influence of the Caliph, first, on politics, and, soon, thereafter also in religious matters. The symbolic importance was, however, increased. All effective power was lost in 946. The Buyyids became the new ruling dynasty, but merely in secular matters. In the following centuries, the role of the Caliph surfaced occasionally, but these were mainly instances in which the secular ruler got the blessings of the Caliph without giving the Caliph any form of influence. The blessings in the shape of a diploma of investiture and robes of honour were given to leaders as strong as Saladin.
In 928 Abdurrahman 3 of Spain, a descendant of the Umayyads, took the title caliph, a title his descendants also carried.
The Fatimids of Egypt had also took this title, as far as back as 909, but they placed less emphasis on this than had the Umayyads of Spain.
When al-Musta'sim was killed in 1258 by the Mongols, he did not leave any heir. The uncle of al-Musta'sim was, however, installed in the position as Caliph in 1261 in Cairo, but he disappeared in the desert while trying to lead an army northward to sack the Mongols.
A new Caliph was installed in 1261, once again in Cairo, this one also a relative of al-Musta'sim. Merely symbolic, without the permission to move freely around, this new line of Caliphs stayed in their position for about 250 years. Except for installing the Sultan in great ceremonies, this Caliph had no importance. The Abbasid Caliph of Cairo was also ignored by the rest of the Muslim world.
In several places, independent Caliphs emerged, whether in Maghreb, with the Seljuqs, the Timurids, the Turkomans, the Uzbeks or the Ottomans.
When the Ottomans conquered Egypt in 1517, the Caliph of Cairo was reinstalled in Constantinople. Shortly after, the new Ottoman Sultan, Selim 2, would call himself Caliph. Later sources would claim that the dignity of the Abbasid caliph had been legitimately transferred to Selim 2.
In the 18th century, the importance of being Caliph had grown stronger for the Ottoman Sultan, who now started to call himself the protector of the Muslim religion. The Ottoman Caliph and Sultan did, in fact, have some influence. With the fall of the Ottoman Empire, the Sultan held on to his title of Caliph for two more years, until his office was abolished in March 1924.
A congress in Cairo in 1926, that tried to reestablish the Caliphate, did not manage to succeed. Important Muslim countries did not participate, and the resolutions agreed upon did not result in real actions, even if they expressed favour for a Caliphate. Since then, nothing has been done, largely due to nationalism in the different countries.
Modern Islamism, as headed by the Muslim Brotherhood, Yusuf al-Qaradawi, Osama bin Laden and others have contented that the reestablishment of the Caliphate is the ultimate goal for their struggle against secularism and Western societies.