Fighters of the Islamic State of Iraq and Syria celebrate on vehicles taken from Iraqi security forces in Mosul, June 12, 2014. Reuters/Stringer

The leader of the Islamic State of Iraq and Syria says his group has achieved its goal of establishing a caliphate, an Islamic state run according to Shariah law. Abu Bakr al-Baghdadi, ISIS chief and now self-proclaimed "Caliph Ibrahim," announced Sunday that his group had fulfilled all the legal requirements for the caliphate and that all existing jihadi groups, including al Qaeda, must swear allegiance to ISIS.

A caliph, according to Islamic scholars, is responsible for governing in terms of Islamic law and is supposed to have the ability to protect the community. The last widely acknowledged caliphate, in Turkey, was abolished in 1924 with the demise of the Ottoman Empire. Al-Baghdadi “is attempting to appeal to an ideal, creating or recreating things the way it should be,” John Esposito, professor of religion and international affairs and of Islamic studies at Georgetown University, said. “[ISIS is] are creating modern, romantic transnational notion of the caliphate. It is a way of legitimating what they are doing.”

The announcement marked the first time the group officially declared governance over the land it has conquered over the last two weeks in Iraq. It now controls the region that stretches from Aleppo in northwestern Syria to Diyala province in northeastern Iraq, on the Iranian border. But, experts said, in order to establish a caliphate in the traditional sense, ISIS would first need to gain support from other Muslims.

Shiites living in ISIS-controlled areas are highly unlikely to pledge allegiance to the group, except under duress, because Shiites by definition believe in a different religious history than Sunnis -- and ISIS considers them heretics. Sunnis look back to the days of the early caliphs as “the great golden age,” said John Voll, former associate director of the Prince Alwaleed bin Talal Center for Muslim-Christian Understanding. The Shiites accept only the fourth caliph, Ali, the son-in-law and cousin of the Prophet Muhammad, and his descendants, the 12 imams.

“This is something that existed historically, when you have a new caliph, the chiefdoms were there to give their allegiance to that caliph,” Esposito said. “This is not a call that is going to appeal to a majority of Muslims in the world. Baghdadi is trying to mobilize fighters and funders, so that he can do what Bin Laden was able to do -- getting money from a variety of people including Gulf businessmen.”

The last incarnation of the caliphate was during the Ottoman Empire; Mustafa Kemal Ataturk, founder of the modern Turkish Republic, abolished it in 1924. Four other caliphates existed before that time, dating back to Muhammad's death in 632. ISIS, a Sunni militant group, wants to establish a state reminiscent of the earliest "rightly guided"caliphs, Esposito said.

Since the late 20th century the caliphate concept has been reappropriated by a number of movements. In particular, Hizb ut-Tahrir, a pan-Islamic political organization founded in 1953 that is led by a Palestinian and has small branches in Europe and Asia, is known for advocating that Muslim countries unify into a caliphate.

“Most people just mean [by the term] that they want something that is authentically Islamic,” Voll said. “But most of the people that make that claim don’t provide any philosophical context.”

Osama bin Laden pushed for global jihad, but was never able to declare an Islamic caliphate. And although other groups have asserted the claim that they established an Islamic caliphate, none of them have controlled territory like ISIS, Voll said.

“Al Qaeda never really ruled anything. ISIS actually rules territory,” he said. “ISIS is much more similar to the Taliban than it is to al Qaeda. The question is, can it maintain law and order and stability?”

ISIS has yet to show it has the capacity to govern -- and al-Baghdadi most likely will not be able to lead the locals living in the towns occupied by ISIS.

“Anyone can claim to be a caliph and becomes it if people pledge allegiance. [But] he has not indicated what would be the institutional structure of the caliphate," Voll said. "There is no bureaucracy in ISIS. It is an ideal and an image rather than an institution."