Late on Sunday, the Isis militant group declared its occupied territories as a new Islamic state, removing "Iraq and the Levant" from its name and announcing the "restoration of the caliphate".
The significance of that final point may have been lost on many in the West since, apart from within the Ahmadiyya sect of Islam, there has not been a caliphate since the days of the Ottoman Empire.
Declaring a "caliphate" is a move that has huge ideological and theological significance. Defined as meaning "the government under a caliph", it means Isis, now simply The Islamic State (IS), has declared its leader Abu Bakr al-Baghdadi as the spiritual leader of all Islam. The word "caliph" comes from the Arabic khalifa, meaning "successor". Its use means the IS claims Baghdadi as the only legitimate successor to the Prophet Mohammed.
Historically, caliphates involved governance under Islamic law, with the leadership elected according to Sunni practice and selected from a group of Imams under Shia traditions.
Laws under a caliphate are traditionally defined in accordance with Islamic ethics. In the past the role of caliph has largely been symbolic, leaving the day-to-day running of government down to the devolved powers of local rulers.
The last widely-acknowledged caliphate was under the Ottoman Empire, which used the symbolic power of its caliph to rule across vast reaches of the Arabic world. The caliphate in this sense ended with the establishment of the Turkish Republic in 1923, though the Ahmadiyya Muslim Community has defined its leader as the "caliph" since the early 20th century.
IS's claim to have restored the caliphate represents a huge challenge to other Islamist and al-Qa'ida-affiliated groups in the region, explains Charlie Cooper, a researcher for the Quilliam counter-extremism think-tank.
He said: "The fact that Isis has done this has huge implications and it is a big challenge to al-Qa'ida, their spokespeople may try to reclaim their legitimacy.