Key questions and answers about the Islamic State group that beheaded American journalist James Foley, and that US President Barack Obama described as a "cancer". What is the Islamic State group?
It emerged in Iraq in 2006, three years after the US-led invasion, spurred by global terror network Al-Qaeda. Initially known as the Islamic State in Iraq.
The jihadists launched deadly attacks on the Shiite majority, oppressed under toppled Sunni dictator Saddam Hussein, and American troops. Sunni tribes rose up against them.
The jihadists rallied to the rebellion in neighbouring Syria in July 2011, joining forces with local rebel groups including Al-Nusra Front, Al-Qaeda's Syria franchise, battling to topple the regime.
In January, deadly clashes erupted between the Iraqi jihadists, now known as the Islamic State of Iraq and the Levant (ISIS), and Al-Nusra and other rebel groups which refused to fight under their banner and instead accused them of atrocities.
In June, ISIS declared an Islamic caliphate in territory it controls in northern Syria and Iraq, renamed itself Islamic State and ordered Muslims to obey its leader, Abu Bakr al-Baghdadi. How many fighters does Islamic State have?
IS recruits through social networks, but jihadists have also joined the group locally out of fear or lured by attractive salaries.
The number of fighters in the group is not known.
But the Syrian Observatory for Human Rights monitoring group says it has more than 50,000 in Syria, including 20,000 jihadists from Chechnya, Europe, China and Gulf Arab countries.
In Iraq, the IS is estimated to number 8,000-10,000 fighters, at least 60% of them Iraqis, according to Ahmad al-Sharifi, a professor of political science at Baghdad University. How much land does IS control?
In Syria it controls about 42% of the country or 45,000 sqkm of territory, while in Iraq it holds sway over 170,000 sqkm or 40%.
Most of this land is desert.
Syria expert Fabrice Balanche says the total amount of territory the IS holds in Syria and Iraq is just under the size of Britain.
In Syria, the "caliphate" announced in June spreads from Manbaj in the northern province of Aleppo, across the northeastern province of Raqa, Hasakeh, to the eastern oil-rich province of Deir Ezzor to the Albu Kamal border town with Iraq.
In Iraq, it controls Sunni regions in the north and west, including second biggest city of Mosul. How does it lure foreign jihadists?
IS has bewitched Western jihadists through the use of spectacular "Hollywood-style" force, brutal beheadings and swift land-grab, says Lebanese writer Hazem al-Amin. How is it financed?
Experts say that through a combination of racketeering, kidnapping for ransom and other criminal activity, as well as donations from wealthy private individuals in the Gulf, the group has built up a financial war chest that is the envy of militant organisations the world over.
Germany's development aid minister Gerd Mueller has accused energy-rich Qatar of financing the jihadists. But Romain Caillet, an expert on Islamists, says funding from the Gulf represents only 5% of its income. What methods does IS use?
A brutal mixture of terror techniques and social services to aid people living under its rule: crucifixion, beheadings and flagellation have all been used by the group against its enemies. Women accused of adultery face death by stoning. What does the future hold for IS?
Caillet believes its key goal is to consolidate the "caliphate", but others, such as Amine, believe that the group is bound to crumble and be forced underground following new and tough Western strikes.