When the extremist group ISIS, the Islamic State in Iraq and Syria, talks about recreating a modern-day caliphate, it makes scholar Tarek Masoud recall how his father described the downfall of the caliphate as a political calamity for Islam.

"I remember as a kid my father telling me that this was the moment of great tragedy for Muslims, that they had once been united and then this office that united them was destroyed," says Masoud, who focuses on the politics of religion and Islam at Harvard's Kennedy School.

The caliphate was the Islamic state established after the death of the prophet Mohammed in the beginning of the seventh century. The caliph, a supreme religious and political leader was at its helm.

"There has been an unbroken chain of caliphs going all the way back from that period ... until 1924, when the caliph was then situated in Istanbul. The caliphate was ended by Mustafa Kemal Atatürk, the leader of Turkey after that country's defeat in the first World War," he notes. "Atatürk basically felt that in order to bring his country, Turkey, into the modern world, it had to get rid of all these old Islamic institutions."

Masoud says the caliphates are especially alluring to extremists today, because they're associated with great Muslim empires.

"Every dynasty that held the caliphate controlled massive tracts of land stretching across continents. This was a period where Muslims really were contenders for great power in the world, and the Muslim empires reached almost to the gates of Europe," he says. "There's also this idea of the caliphate, particularly in the caliphates' early years — the first 30 years of the caliphate — of it embodying the purist, most just government that Muslims have ever experienced."

Masoud says historical caliphates, like the 8th century caliphate that offered the setting for 1001 Arabian Nights, differ dramatically from the sorts of institutions that ISIS envisions.

"There was wine and drinking and poetry, and it was very different from what ISIS has in mind," he says. "ISIS, I think, is part of this intellectual tradition in the Muslim world to restore the unity of the Muslims ... But it doesn't just want Muslim unity. It also has a vision for how Muslims should be governed. And that's by the very strict application of a very retrograde form of sharia law."

Masoud says today, the caliphate isn't something most Muslims think about on a daily basis.

"It's probably something more relevant for people of my father and grandfather's generation," he notes. "The majority of Muslims and the majority of Arabs generally accept the legitimacy of the nation-states that they inhabit."