It is a force the like of which the world has never seen before: a medieval death cult with a territory roughly the size of England. Its tentacles of power stretch out across Iraq and Syria, whose border it has erased. Syrian Raqqa is the de facto capital, Iraqi Mosul its most populous centre. Within its brutal control lies a huge population of eight million – and it has assets estimated at $2 trillion. This is the  Islamic State, sometimes called ISIS, a terrorist movement with the destructive power of an army possessed by a terrifying vision of the world to come.

ISIS started as a version of Al Qaeda, but had broken off its allegiance by 2011. It arranged mass prison breakouts in Iraq in 2012. which helped swell its numbers by 10,000 but it wasn’t until 2014 that it made any gains, when it suddenly took the Iraqi cities of Fallujah, Ramadi and Mosul in quick succession. Since then its numbers have grown to 33,000 thanks to foreign recruits.

THE BIRTH OF A MONSTER IS started as a version of Al Qaeda in Iraq, which resisted American occupation after 2003 and then benefited from the mainly Shia government’s exclusion of the country’s large Sunni minority. The early versions of the group experimented establishing a rigorous Sharia state. The Syrian civil war was its next big chance, when after 2011 a popular uprising became an insurgency involving many armed groups. By then IS had broken with Al Qaeda. In 2012 it organised eight mass prison breaks in Iraq, swelling its numbers to 10,000. That force launched rapid conquests in 2014, taking the Iraqi cities of Fallujah, Ramadi and Mosul in quick succession. No one saw it coming. It has since attracted thousands of foreign volunteers, swelling to 33,000 fighters, according to the CIA.

From the start, IS combined religious zealots with cynical, whisky-drinking former servants of Saddam Hussein’s Ba’ath Party. They grew to know one another in American holding centres and kept in contact with phone numbers written on the elastic of their underwear. The overall leader is 40-year-old ‘Caliph Ibrahim’ a former theology student called Awwad Ibrahim al-Badri, better known as Abu Bakr al-Baghdadi. Numerous former Ba’athist military commanders and security police experts have lurked in the IS shadows, many now killed by drones. Its organisational structures were drawn up by a former Syrian air force colonel, Haji Bakr, who rigged Badri’s election as Caliph in 2010. His intelligence background means he knows how to keep an iron grip on conquered populations. The current commanders are Abu Suleiman al-Naser and, in the field, a orgian Chechen jihadist (a former army sergeant partly trained in Georgia by the Americans) called Abu Omar al-Shistani. How they exist alongside religious zealots is a mystery.