Since the 9/11 attacks, there has been much western interest in Islam. Barack Obama, Tony Blair, George Bush and many others have all spoken of the religion as one "of peace." Others have argued the opposite. But the debate about the nature of the faith and its relation to violent extremism is missing an important element. If we want to understand the world view and aims of IS, and why some people seem attracted to its project, we would do better to focus more on the history of Islam, both as understood by militants and as it actually occurred, than the extraordinarily difficult question of the essential nature of a religion.

One obviously important area of historical inquiry is the life of Mohammed. Dozens of books have been published explaining the centrality of the Prophet within Islam, as well as the consequences of the the various imperialist incursions and occupations in the Islamic world since the 19th century. But the 11 centuries between the death of Mohammed in 632 and the arrival of Napoleon in Egypt in 1798 have received less attention outside specialist circles. This is a shame because much of what is happening now can be explained by what happened then--particularly IS's project. It may allow us some, very qualified, optimism about the long-term prospects for the group.

It was not Mohammed himself but his first four successors who oversaw the campaigns that turned Islam from a new creed restricted to the Arabian Peninsula into a global imperial force. Around half of the historical references cited by al-Adnani in his statement in March occurred during the rules of the caliphs Abu Bakr, Omar, Othman and Ali from 632 until 661. In his book Destiny Disrupted: A History of the World Through Islamic Eyes, the Afghan-American writer Tamim Ansary argues that the core religious allegory of Islam--analogous to exodus, bondage and the return to the promised land for the Jews, or the last supper, crucifixion and resurrection of Christ for Christians--is not limited to Mohammed's life, but includes the reigns of these men too.

Quite why Islam spread as fast as it did is still debated. Some historians suggest it was the military superiority of the early Arab armies that was primarily responsible. The black flags under which contemporary extremists fight, and which they use as idents on their videos and fly above their offices in places like Raqqa, deliberately recall what are imagined to be the battle banners of the earliest Muslim forces. Those troops' historic success may have been due to extremely capable battlefield leaders, the faith of the fighters, their ability to do without cumbersome supply trains, or flexible and innovative tactics. It may also have been because the faith emerged at a time when the two superpowers of the era--Byzantine Rome and the Persians--had exhausted themselves in centuries of conflict.

The conquests meant that Muslims' collective memory has a different starting point from that of Jews or Christians. Mohammed did not merely outline a vision of a utopian community to be realised at an unspecified future date but actually built one during his lifetime. That community then transformed much of the known world, through diplomacy, trade, cultural exchange and war. While for Jews the collective memory of the earliest believers is exile, and for Christians persecution, for Sunni Muslims at least, it is one of the most successful military and political campaigns in history.

Moreover, as its great cities expanded and its traders prospered, the new Islamic empire developed into a hugely rich and powerful civilisation. The Umayyads, who ruled from the death of Ali in 661 to 750 from Damascus, continued to acquire new territory, extending their rule as far as the Atlantic coast of the Iberian peninsula to the west and the Indus valley in the east. They gave the new imperial entity a permanence in other ways too. Some of the most famous examples of Islamic architecture--the Great Mosque of Damascus, the al-Aqsa Mosque in Jerusalem--date from this period.

The Abbasids, who overthrew the Umayyads in 750, ruled from a series of cities including Baghdad, Raqqa and Samarra, and are credited with ushering in a golden age of Islamic civilisation. By the turn of the first millennium, the new empire had splintered into states run by competing dynasties, but brilliant cultural activity continued, and the various incursions of the Crusaders from the west were eventually repulsed and invasions from the east successfully resisted. Even the catastrophic sack of Baghdad by the Mongols in 1258 did not mean the era of the great Islamic rulers was over. Those who had destroyed the great city converted to Islam themselves. Within 200 years, Constantinople fell to the Ottoman Turks, who went on to conquer much of the Balkans and threaten central Europe. Even as late as the 17th century, no European state, with the arguable exception of Catholic Spain, came close to rivalling the Ottoman empire's territorial extent, military capability, scientific knowledge and artistic achievement. From Delhi, the Mughals, an Islamic dynasty descended from Mongol converts, dominated south Asia. Their wealth and power were fabulous. Between these two superpowers, the Safavids built their spectacular Shia state in Persia. The contrast with the poor, backward, bickering, strife-torn nations of Europe is striking.

What today's commentators in London and Washington often forget--and militants repeatedly remind themselves and anyone else prepared to listen--is that the supremacy of the west is a relatively new phenomenon in historical terms. Across much of the world, for two thirds of the last 1,300 years, the power, the glory and the wealth was, broadly speaking, Islamic. The story of the caliphate, both as historical reality and as imagined by extremists like those of the Islamic State, can only be understood within the context of this overarching narrative, as the means by which the militants seek to return the world's Muslim community to what it sees as its rightful status: a global superpower. [...]

First and foremost, the caliphate would allow Muslims to heal the damage done by centuries of western dominance, through dismantling all the structures it had imposed. "The Muslims were defeated after the fall of their caliphate," al-Baghdadi wrote. "Then their state ceased to exist, so the unbelievers were able to weaken and humiliate the Muslims, dominate them in every region, plunder their wealth and resources, and rob them of their rights. They accomplished this by attacking and occupying their lands, placing their treacherous agents in power to rule the Muslims with an iron fist, and spreading dazzling and deceptive slogans such as civilisation, peace, coexistence, freedom, democracy, secularism, Baathism, nationalism and patriotism, among other falsehoods."