29 June 2015
So-called Islamic State burst on to the international scene in 2014 when it seized large swathes of territory in Syria and Iraq. It has become notorious for its brutality, including mass killings, abductions and beheadings. The group though has attracted support elsewhere in the Muslim world - and a US-led coalition has vowed to destroy it.
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 allegiance to its leader - Ibrahim Awad Ibrahim al-Badri al-Samarrai, better 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 already have, among them several offshoots of the rival al-Qaeda network.
IS seeks to eradicate obstacles to restoring God's rule on Earth and to defend the Muslim community, or umma, against infidels and apostates.
The group has welcomed the prospect of direct confrontation with the US-led coalition, viewing it as a harbinger of an end-of-times showdown between Muslims and their enemies described in Islamic apocalyptic prophecies .
What are its origins?
IS can trace its roots back to the late Abu Musab al-Zarqawi, a Jordanian who set up Tawhid wa al-Jihad in 2002. A year after the US-led invasion of Iraq, Zarqawi pledged allegiance to Osama Bin Laden and formed al-Qaeda in Iraq (AQI), which became a major force in the insurgency.
Image caption The tactics of Abu Musab al-Zarqawi were considered too extreme by al-Qaeda leaders
After Zarqawi's death in 2006, AQI created an umbrella organisation, Islamic State in Iraq (ISI). ISI was steadily weakened by the US troop surge and the creation of Sahwa (Awakening) councils by Sunni Arab tribesmen who rejected its brutality.
Baghdadi, a former US detainee, became leader in 2010 and began rebuilding ISI's capabilities. By 2013, it was once again carrying out dozens of attacks a month in Iraq.
It had also joined the rebellion against President Bashar al-Assad in Syria, setting up the al-Nusra Front.
In April 2013, Baghdadi announced the merger of his forces in Iraq and Syria and the creation of the Islamic State in Iraq and the Levant (Isis). The leaders of al-Nusra and al-Qaeda rejected the move, but fighters loyal to Baghdadi split from al-Nusra and helped Isis remain in Syria.
Image caption Religious minorities, particularly Iraq's Yazidis, have been targeted by Islamic State
At the end of December 2013, Isis shifted its focus back to Iraq and exploited a political stand-off between the Shia-led government and the minority Sunni Arab community. Aided by tribesmen and former Saddam Hussein loyalists, Isis took control of the central city of Falluja.
In June 2014, Isis overran the northern city of Mosul, and then advanced southwards towards Baghdad, massacring its adversaries and threatening to eradicate the country's many ethnic and religious minorities. At the end of the month, after consolidating its hold over dozens of cities and towns, Isis declared the creation of a caliphate and changed its name to Islamic State.
How much territory does IS control?
Image caption Some Sunni Arabs showed their support for Islamic State after the group overran Mosul
In September 2014, the director of the US National Counterterrorism Center said IS controlled much of the Tigris-Euphrates river basin - an area similar in size to the United Kingdom, or about 210,000 sq km (81,000 sq miles).
Seven months later, the US military declared that IS had lost about a quarter of its territory in Iraq - equating to 13,000 to 15,500 sq km - but that its area of influence in Syria remained largely unchanged, with losses in some areas offset by gains in others.
However, these figures do not necessarily reflect the situation on the ground. In reality, IS militants exercise complete control over only a small part of that territory, which includes cities and towns, main roads, oil fields and military facilities.
They enjoy freedom of movement in the largely uninhabited areas outside what the Institute for the Study of War calls "control zones", but they would struggle to defend them.
Similarly, 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 put the figure at more than 10
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.
How many fighters does it have?
Image caption Thousands of foreigners have fought for Islamic State in Syria and Iraq
In February 2015, US Director for National Intelligence James Clapper said IS could muster "somewhere in the range between 20,000 and 32,000 fighters" in Iraq and Syria.
But he noted that there had been "substantial attrition" 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 10,00 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 May 2015, a state department official estimated that the group had attracted more than 22,000 foreign fighters from more than 100 countries.
Studies by the London-based International Centre for the Study of Radicalisation and Political Violence (ICSR) and the New York-based Soufan Group suggest that while about a quarter of the foreign fighters are from the West, the majority are from nearby Arab countries, such as Tunisia, Saudi Arabia and Jordan and Morocco.
What weapons does IS have?
Image caption Islamic State has become one of the most formidable jihadist groups in the world
IS fighters have access to, and are capable of using, a wide variety of small arms and heavy weapons, including truck-mounted machine-guns, rocket launchers, anti-aircraft guns and portable surface-to-air missile systems.
They have also captured tanks and armoured vehicles from the Syrian and Iraqi armies. Their haul of vehicles from the Iraqi army includes armoured Humvees and bomb-proof trucks originally manufactured for the US military.
Some have been packed with explosives and used to devastating effect in suicide bomb attacks.
The group is believed to have a flexible supply chain that ensures a constant supply of ammunition and small arms for its fighters. Their considerable firepower helped them overrun Kurdish Peshmerga positions in northern Iraq in August 2014 and the Iraqi army in Ramadi in May 2015.
Where does IS get its money from?
Image caption In areas under its control, Islamic State controls trade and collects taxes and fees
The militant group is believed to be the world's wealthiest. It initially relied on wealthy private donors and Islamic charities in the Middle East keen to oust Syria's President Assad. Although such funding is still being used to finance the travel of foreign fighters to Syria and Iraq, the group is now largely self-funding.
The US Treasury estimates that in 2014 IS may have earned as much as several million dollars per week, or $100m in total, from the sale of crude oil and refined products to local middlemen, who in turn smuggled them in Turkey and Iran, or sold them to the Syrian government.
But air strikes on oil-related infrastructure are now believed to have diminished such revenue.
Kidnapping also generated at least $20m in ransom payments in 2014, while IS raises several million dollars per month through extorting the millions of people living in areas under its full or partial control, according to the US Treasury.
IS is believed to raise at least several million dollars per month by robbing, looting, and extortion. Payments are extracted from those who pass through, conduct business in, or simply live in IS territory under the auspices or providing services or "protection".
Religious minorities are forced to pay a special tax. IS profits from raiding banks, selling antiquities, and stealing or controlling sales of livestock and crops. Abducted girls and women have meanwhile been sold as sex slaves.
Why are their tactics so brutal?
Image caption Videos and photographs of beheadings have helped persuade thousands of soldiers to abandon their posts
IS members are jihadists who adhere to an extreme interpretation of Sunni Islam and consider themselves the only true believers. They hold that the rest of the world is made up of unbelievers who seek to destroy Islam, justifying attacks against other Muslims and non-Muslims alike.
Even al-Qaeda leader Ayman al-Zawahiri, who disavowed IS in February 2014 over its actions in Syria, warned Zarqawi in 2005 that such brutality loses "Muslim hearts and minds".