The Islamic State of Iraq and the Levant, or ISIL, renamed itself simply as the Islamic State, and ordered the world's Muslims to obey Abu Bakr al-Baghdadi, now the "caliph" or successor to the Prophet Mohammed.
What is a caliphate?
After the Prophet Mohammed died in 632 AD, his followers agreed on the caliphate system, meaning succession in Arabic, as the new mode of rule.
The caliph's main duty was to implement Muslim law in the land of Islam and spread it across an empire that expanded from what is now western Saudi Arabia.
The first caliph was appointed in two stages, under which representatives of Muslim communities chose Abu Bakr and then submitted his name to the public seeking their backing.
But from day one Muslims differed on the concept of the caliphate -- a mainly Sunni system that Shiites contest as they believe the cousin of the prophet, Imam Ali, and his offspring had a divine right to lead after Mohammed's death.
Under the caliphate, a whole governing structure was developed and expanded as the territory of the state stretched in all directions. The caliph had ministers and appointed rulers in the widespread emirates.
How long did it last?
Zealous Muslims believe the caliphate lasted until it was abolished following the defeat of the Ottoman Empire in World War I.
But it is generally thought to have lasted in its original form for just three decades, during the reign of the first four leaders, known as the Four Rightly Guided Caliphs.
Several dynasties fought for power and ruled in the empire's vast territories, including the Omayyads in Damascus (661-750), Abbasids in Baghdad (750-1258), and the Ottomans in Turkey (1453-1924).
The succession process was hereditary and rulers all adopted the title of caliph.
In March 1924, the founding father of modern Turkey, Mustafa Kemal Ataturk, constitutionally abolished the caliphate.
What were its boundaries?
The land where Islam flourished has never been limited by a constitution, and expansion to spread the religion was always considered part of the caliphate's role.
At its peak, Ottoman rule covered the Middle East and North Africa, the Caucasus and parts of Eastern Europe.
Are any movements calling for the revival of a caliphate?
Political Islam in general calls for the rule of sharia law as a system of life, including politics.
The founder of the Egypt's Muslim Brotherhood, Hasan al-Banna, considered the caliphate a symbol of Islamic unity, and reinstating the system was one of its goals. But Banna argued the caliphate needed to be preceded by cooperation agreements between Muslim states to pave the way for an eventual union led by an agreed-upon imam.
Hizb ut-Tahrir, or the Party of Liberation, a pan-Islamic group formed in 1953, is known for focusing on unifying Muslim countries in a caliphate.
Did Al-Qaeda want a caliphate?
An Islamic state has been the "great dream" of Al-Qaeda since the 11 September, 2001 attacks on the United States, according to Mustafa al-Ani, from the Gulf Research Centre.
ISIL's declaration is only a "nucleus for the caliphate that would expand with the collapse of established states," he argued.
In 1996, the Taliban established an Islamic emirate in Afghanistan which it ruled until the jihadist group was ousted by a US-led offensive in 2001. But the leader of the Taliban did not adopt the caliph title, preferring the rank of Amir al-Muminin, or commander of the faithful, another title carried used by caliphs.
Does the caliphate have a future?
The Islamic State could remain in place "in the current situation which is characterised by the weakness of the government in Baghdad and the absence of a foreign intervention," said Ani.
But the jihadists would have to "liquidate other Islamist groups" not loyal to them and crush any attempt at revolt within the territories they control. They would also have to strengthen their defences and impose the rule of Islamic courts.