Nostalgia is powerful, something Abu Bakr al-Baghdadi, aka Caliph Ibrahim, clearly gets. He heads the Islamic extremist group that’s conquered big chunks of Iraq and Syria.

Al-Baghdadi declared the creation of a new Islamic caliphate earlier this month, evoking the medieval golden age of the Muslim world, when Islamic leaders ruled an empire that at one point stretched from Morocco and Spain in the west to Afghanistan in the east.

The caliphate was the political structure ruling this enormous empire, as followers of the Prophet Muhammad expanded their domain after his death in the 7th century AD. The caliphate was “a new form of Middle Eastern empire defined and legitimized in Islamic terms,” explains Ira Lapidus in his book, A History of Islamic Societies.

Now, [the caliphate] represents an idealized image of Islamic ascendancy more than a particular form of governance.

The true era of the caliphate only lasted about 300 years — until the middle of the 10th century — undermined by internal rivalries. Genghis Khan and his Mongol hordes put the final nail in the coffin when they sacked Baghdad in the 13th century. The Ottomans sought to rekindle the tradition of the caliphate in the 18th and 19th centuries to assert rule from Constantinople (today’s Istanbul) over vast tracts of the Islamic world. But it was more symbol than reality.

Still, successive generations of Muslims have tried to reclaim the mantle since the caliphate’s heyday.

“What we’re really talking about is the history of an idea that waxed and waned and waned pretty quickly,” says L. Carl Brown, a historian who specializes in the Arab world and professor emeritus at Princeton University. Now, more than anything, it represents an idealized image of Islamic ascendancy more than a particular form of governance.

Really?