What is IS(Islamic state)?
The jihadist group Islamic State (IS) 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?
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. In 2004, 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.
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.
How much territory does IS control?
In September 2014, the then director of the US National Counterterrorism Center (NCTC), Matthew Olsen, 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).
A year later, the US defence department declared that IS frontlines in much of northern and central Iraq and northern Syria had been pushed back significantly by US-led coalition air strikes and ground operations. IS could no longer operate freely in roughly 20-25% of populated areas in Iraq and Syria where it once could, it said.
How many fighters does it have?
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 10,000 IS fighters had been killed.
What about its targets outside Iraq and Syria?
In late 2015, IS began to lay claim to attacks outside its territory. An Egyptian affiliate, Sinai Province, said it downed a Russian passenger plane in the Sinai peninsula, killing all 228 on board. It gave no details, but the UK and US later said it was likely that a bomb caused the crash - whether or not a bomber was linked to IS.
What weapons does IS have?
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.
Where does IS get its money from?
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.
Why are their tactics so brutal?
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.
Beheadings, crucifixions and mass shootings have been used to terrorise their enemies. IS members have justified such atrocities by citing the Koran and Hadith, but Muslims have denounced them.
