The Islamic State of Iraq and the Levant (Isil), also known as simply Islamic State, began life as an Iraqi franchise of al-Qaeda, a relatively minor force confined to fighting British and United States troops in the areas around Baghdad.
The terrorist group, which recently released a video in which a British recruit appears to behead kidnapped US journalist James Foley, broke ties with al-Qaeda and rebranded as ISIS in April 2013. ISIS then took advantage of the chaos caused by Syria’s ongoing civil war to expand beyond Iraq’s borders and recruit vast numbers of Syrian rebels.
IS, as it now calls itself, went on to a string of successful missions in March, wresting control of the Syrian city of Raqqa from Bashar al-Assad’s army.
In January 2014 it took control of the Iraqi city of Fallujah, in the western province of Anbar, and made significant gains in the provincial capital Ramadi.
Then in June Baghdadi’s fighters, by now swelled by thousands of new recruits, dealt a spectacular blow to Baghdad’s Shiite-led government by taking control of Iraq’s second city Mosul.
The takeover of Mosul prompted the US to voice deep concern about the “extremely serious” situation and warn the jihadist Sunni group poses “a threat to the entire region”.
By August, IS’s lightning advances across Iraq and Syria pushed the US to order air strikes in support of Kurdish peshmerga forces and the Iraqi army, leading to recapture of the strategically vital Mosul dam from IS.
IS is led by Abu Bakr al-Baghdadi, the self-styled Caliph of the new Islamic State.
Little is known about Baghdadi, who is revered by IS recruits, but he is thought to have joined the insurgency that erupted in Iraq soon after the 2003 US-led invasion.
In 2010 he emerged as leader of Al-Qaeda in Iraq before becoming leader of breakaway IS.
Western governments fear the group could eventually emulate al-Qaeda and strike overseas, but their biggest worry for now is likely the eventual return home of foreign fighters attracted by ISIS and Baghdadi. In June 2014 a video emerged showing British fighters appealing to other westerners to join the jihad in Iraq.
IS has attracted recruits far beyond the borders of Iraq and Syria.
Among them are men like Mehdi Nemmouche, a 29-year-old Frenchman who allegedly carried out a deadly shooting on a Jewish museum in Belgium last month after spending a year fighting with ISIS in Syria.
The Soufan Group, a New York-based consultancy, estimates that 12 000 foreign fighters have travelled to Syria, including 3 000 from the West.
And IS appears to have the greatest appeal, with King’s College London Professor Peter Neumann estimating around 80% of Western fighters in Syria have joined the group.
Unlike other groups fighting Assad, ISIS is seen working towards an ideal Islamic emirate that straddles Syria and Iraq. And compared with al-Qaeda’s franchise in Syria, Al-Nusra Front, it has lower entry barriers.
IS has also sought to appeal to non-Arabs, recently publishing two English-language magazines, having already released videos in English, or with English subtitles.
It claims to have had fighters from Britain, France, Germany and other European countries, as well as the US, and from the Arab world and the Caucasus.
Originally funded by wealthy donors, the group is now thought to derive significant income from captured oil fields in northern Iraq and Syria.
IS fighters also reportedly stole £256 million in cash and a large amount of gold bullion from Mosul’s central bank during its takeover of the city, and reportedly smuggled £21m of antiquities from the Syria.
As well as funds IS is now in possession of a large amount of US military equipment, donated to the Iraqi army as the US pulled out of Iraq and seized by IS as it captured territory from Iraqi forces.
Much of the appeal of IS for recruits stems from Baghdadi himself – the IS leader is touted as a battlefield commander and tactician, a crucial distinction compared with al-Qaeda chief Ayman al-Zawahiri.
“For the last 10 years or more, (Zawahiri) has been holed up in the Afghanistan-Pakistan border area and hasn’t really done very much more than issue a few statements and videos,” said Richard Barrett, a former counter-terrorism chief at MI6, Britain’s foreign intelligence service.
“Whereas Baghdadi has done an amazing amount – he has captured cities, he has mobilised huge amounts of people, he is killing ruthlessly throughout Iraq and Syria.
“If you were a guy who wanted action, you would go with Baghdadi,” Barrett told AFP.
In October 2005, American forces said they believed they had killed “Abu Dua”, one of Baghdadi’’s known aliases, in a strike on the Iraq-Syria border.
But that appears to have been incorrect, as he took the reins of what was then known as the Islamic State of Iraq, or ISI, in May 2010 after two of its chiefs were killed in a US-Iraqi raid.
Since then, details about him have slowly trickled out.
In October 2011, the US Treasury designated him as a “terrorist” in a notice that said he was born in the Iraqi city of Samarra in 1971.
And earlier this year, Iraq released a picture they said was of Baghdadi, the first from an official source, depicting a balding, bearded man in a suit and tie.
At the time Baghdadi took over, his group appeared to be on the ropes, after “the surge” of US forces combined with the shifting allegiances of Sunni tribesmen to deal him a blow.
But the group bounced back, expanding into Syria in 2013.
Baghdadi sought to merge with Al-Nusra, which rejected the deal, and the two groups have operated separately since.
Zawahiri has urged ISIS to focus on Iraq and leave Syria to Al-Nusra, but Baghdadi and his fighters have openly defied the al-Qaeda chief and, indeed, have fought not only Assad, but also Al-Nusra and other rebel groups.