The Rise of ISIS
The group's history and its activity
Its precise size is unclear but it is thought to include about 40000 fighters, including many foreign jihadists.
The organisation is led by Abu Bakr al-Baghdadi. Little is known about him, but it is believed he was born in Samarra, north of Baghdad, in 1971 and joined the uprising that started in Iraq soon after the invasion of the US in 2003.
In 2010 he emerged as the leader of al-Qaeda in Iraq, one of the groups that later became Isis.
Prof Peter Neumann of King's College London estimates that about 80% of Western fighters in Syria have joined the group.
The group has seen great military success. In March 2013, it took over the Syrian city of Raqqa - the first capital to fall under rebel control.
It also captured large sections of the provincial capital, Ramadi, and has a presence in a number of towns near the Turkish and Syrian borders.
The group has a reputation for brutal rule in the areas that it controls. However, the conquest of Mosul in June sent shockwaves around the world.
Initially, the group relied on donations from rich people in Gulf Arab states, particularly Kuwait and Saudi Arabia, who supported its fight against President Bashar al-Assad.
Today, it is believed IS earns significant amounts of money from the oil fields it controls in eastern Syria. It is also believed to have been selling looted antiquities from historical sites.
Prof Neumann believes that before the capture of Mosul in June 2014, IS had cash and assets worth about $900m . Afterwards, this rose to around $2bn.
The group has been operating independently of other jihadist groups in Syria such as the al-Nusra Front, the official al-Qaeda affiliate in the country, and has had a tense relationship with other rebels.
Hostility to IS grew steadily in Syria and in January 2014, rebels from both Western-backed and Islamist groups launched an attack against IS, seeking to drive its foreign fighters out of Syria. Thousands of people are reported to have been killed in the fighting.
What does IS want?
In June 2014, the group formally declared the establishment of a "caliphate" - a state governed in accordance with Islamic law, or Sharia, by God's deputy on Earth, or caliph.
It has demanded that Muslims across the world swear to be loyal to its leader - Ibrahim Awad Ibrahim al-Badri al-Samarrai, also known as Abu Bakr al-Baghdadi - and migrate to territory under its control.
IS has also told other jihadist groups worldwide that they must accept its supreme authority. Many have already accepted, among them several branches of the rival al-Qaeda network.
It is not entirely clear how many people are living under full or partial IS control across Syria and Iraq. In March 2015, the president of the International Committee of the Red Cross estimated them at more than 10 million.
Inside areas where IS has implemented its strict interpretation of Sharia, women are forced to wear full veils, public beheadings are common and non-Muslims are forced to choose between paying a special tax, converting or death.
In February 2015, US Director for National Intelligence James Clapper said IS have "somewhere in the range between 20,000 and 32,000 fighters" in Iraq and Syria.
But he noted that there had been substantial weakening in its ranks since US-led coalition air strikes began in August 2014. In June 2015, US Deputy Secretary of State Antony Blinken said more than 10,000 IS fighters had been killed.
To help mitigate the manpower losses, IS has turned to conscription in some areas. Iraqi expert Hisham al-Hashimi believes only 30% of the group's fighters are "ideologues", with the remainder joining out of fear or coercion.
A significant number of IS fighters are neither Iraqi nor Syrian. In October 2015, National Counterterrorism Center Director Nicholas Rasmussen told Congressthat the group had attracted more than 28,000 foreign fighters. They included at least 5,000 Westerners, approximately 250 of them Americans, he said.