AP
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. 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.
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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 "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.
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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".
