The Islamic State in Iraq and Syria (ISIS) is arguably the most dangerous terrorist threat faced by the world today.
Not only has it rapidly gained territory in Iraq and Syria, it is gaining adherents around the world with its message of extremism.
The brutal terror organisation, which rose to global prominence in 2014, has its roots in Iraq more than a decade earlier.
Here is a quick look at the extremist group:
1. The origins of ISIS
The Islamic State in Iraq and Syria (ISIS) began as the Al-Qaeda in Iraq (AQI) in the early 2000s.
The organisation was originally led by Abu Musab al-Zarqawi, a Jordanian and a convicted thief before he turned to radical Islam and formed his group of fighters drawn from Iraq and a region known in Arabic as the "al-Sham" or the Levant, that is Palestine, Lebanon and Syria.
The group was known for its savagery and targeting of Shi'ites in Iraq, alienating many Iraqis and Al-Qaeda leaders.
Zarqawi was killed in an US airstrike in 2006, and AQI joined other militant groups to form the Islamic State of Iraq.
The outbreak of war in Syria provided fresh opportunity for the group.
2. The rise of ISIS
From 2010, the group's emir was Abu Bakr al-Baghdadi, who claims to be a direct descendant of Prophet Mohammed.
As uprisings in Syria began in 2011, the group joined in the fight against the government. Its ruthless tactics alienated other rebel groups, but it gained many new fighters and valuable battle experience.
It also launched strategic attacks to seize resources such as arms caches, oil wells and granaries in Syria.
A colonel from Saddam Hussein's disbanded military drew up the plans for ISIS's expansion in Syria, according to a Der Spiegel report in April 2015.
The story argued that the strategic calculations of a group of former Iraqi intelligence officers combined with the fanatical beliefs of the group led to its success.
In 2013, the group began calling itself ISIS, or the Islamic State in Iraq and the Levant (ISIL), and later, simply the Islamic State (IS).
The group was formally ejected from Al-Qaeda in 2014 as Baghdadi defied orders from Al-Qaeda's leadership to withdraw from Syria and go back to Iraq.
3. Establishing a caliphate
The group's goal since 2004 is to establish a hardline Sunni Islamic state.
In a major breakthrough, it captured the Iraqi city of Mosul in June 2014. The militant group immediately announced it was establishing a caliphate, or Islamic state, on the territories it controlled.
Baghdadi declared himself leader of the state, and that he would be known as "Caliph Ibrahim". As caliph, he is both the political and religious authority.
4. Terror and territory
As of May 2015, ISIS controls Iraq's Anbar province and its regional capital Ramadi. It also holds Mosul, and other cities.
While it was forced to give ground in Tikrit in April 2015, Saddam Hussein's home town, the group still controls large areas of Iraq.
It is said to be holding sway half of Syria's landmass in May after seizing Palmyra, a historic city which was once a Silk Road hub.
The group's stronghold is in Raqqa, where it has administrative control, and it also holds large parts of Aleppo and Deir Ezzor provinces.
It has direct control over key slivers of populated regions, and it operates with impunity in the vast Homs desert in Syria, which also provides insulation against other militant forces.
5. Oil and revenues
ISIS is called the best-financed terrorist group ever known. It reportedly rakes in millions of dollars a month, which it uses to pay its combatants and buy weaponry.
Reports have said that it controls about half a dozen oil fields. Besides revenue from oil smuggling, the group receives money through donations from wealthy sympathisers.
It also seizes money and assets in each area it takes over, then collects taxes and tolls as it establishes its control.
6. Acts of atrocity
Reports of mass killings have surfaced in most of the places ISIS has invaded. Days after taking over Palmyra, fighters from the group were said to have executed at least 400 people, mostly women and children.
It shocked the world with videos of the beheadings of hostages, including Japanese, American and British citizens.
A United Nations report in March 2015 said that ISIS has committed war crimes and crimes against humanity including torture, rape, sexual slavery and the use of child soldiers. They have also launched systematic attacks against the Yazidi minority in Iraq and Syria which veer on genocide.
7. Foreign fighters
Despite, or perhaps because of its brutality, the extremist group has drawn a large number of fighters from abroad.
A United Nations report in April 2015 said that some 20,000 foreign fighters from over 100 countries have travelled to join the militant groups fighting in Iraq and Syria, most of these flowing to ISIS.
The flow of foreign fighters is higher than it has ever been historically, experts said.
2004: Abu Musab al-Zarqawi establishes Al-Qaeda in Iraq (AQI).
2006: Under al-Zarqawi, Al-Qaeda in Iraq tries to ignite a sectarian war against the majority Shi'ite community.
June 7, 2006: Al-Zarqawi was killed in a US strike. Abu Ayyub al-Masri, also known as Abu Hamza al-Muhajer, takes his place as leader of AQI.
October 2006: AQI leader Abu Ayyub al-Masri announces the creation of Islamic State in Iraq (ISI), and establishes Abu Omar al-Baghdadi as its leader.
April 2010: Abu Bakr al-Baghdadi becomes leader of ISI after Abu Omar al-Baghdadi and Abu Ayyub al-Masri were killed in a joint US-Iraqi operation.
April 8, 2013: ISI declares its absorption of an Al-Qaeda-backed militant group in Syria, Jabhat al-Nusra, also known as the al-Nusra Front. Al-Baghdadi says that his group will now be known as ISIS.
April 2013: Al-Nusra Front leader Abu Mohammed al-Jawlani rejects ISIS's attempt to merge with the group.
Feb 3, 2014: Al-Qaeda renounces ties to ISIS after months of infighting between al-Nusra Front and ISIS.
June 9, 2014: Monday night into Tuesday, militants seize Mosul's airport, its TV stations and the governor's office. ISIS frees up to 1,000 prisoners.
June 10, 2014: ISIS takes control of Mosul.
June 11, 2014: ISIS takes control of Tikrit.
June 21, 2014: ISIS takes control of Al-Qaim, a town on the border with Syria, as well as three other Iraqi towns.
June 29, 2014: ISIS announces the creation of a caliphate (Islamic state) that erases all state borders, making al-Baghdadi the self-declared authority over the world's estimated 1.5 billion Muslims. The group also announces a name change to the Islamic State (IS).
July 2014: In Syria, all the cities between Deir Ezzor city and the Iraq border have fallen to ISIS, says Omar Abu Leila, a spokesman for the rebel Free Syrian Army.
July 3, 2014: ISIS takes control of a major Syrian oil field, al-Omar.It is the country's largest oil field and can produce 75,000 barrels of oil daily.
July 17, 2014: In Syria's Homs province, ISIS claims to have killed 270 people after storming and seizing the Shaer gas field.
July 24, 2014: ISIS militants blow up Jonah's tomb, a holy site in Mosul.
Aug 8, 2014: Two US F/A-18 jet fighters bomb artillery of Sunni Islamic extremists in Iraq. President Barack Obama has authorised "targeted airstrikes" if needed to protect US personnel from fighters with ISIS. The US military also could use airstrikes to prevent what officials warn could be a genocide of minority groups by the ISIS fighters.
Aug 19, 2014: In a video posted on YouTube, US journalist James Foley, missing in Syria since 2012, is decapitated by ISIS militants. The militants then threaten the life of another captured US journalist, believed to be Steven Sotloff.
Sept 2, 2014: ISIS releases a video showing the beheading of US journalist Steven Sotloff. Sotloff's apparent executioner speaks in what sounds like the same British accent as the man who purportedly killed Foley. He is dressed identically in both videos, head to toe in black, with a face mask and combat boots. He appears to be of similar build and height. He waves a knife in his left hand, as did the militant in the video of Foley's death.
Sept 10, 2014: Obama announced expanded air strikes in Iraq, part of a broader strategy to combat Islamic militants in Iraq and Syria. He also offered the first hint that he would be willing to launch similar strikes in Syria, even as he stressed Washington's continued opposition to Syrian leader Bashar al-Assad.
Sept 13, 2014: ISIS militants post video on a website associated with the group, showing the apparent execution of British aid worker David Haines.
Sept 23, 2014: The US and its five Middle East allies moved the Obama plan to battle ISIS into a new phase, as waves of drones and bombers, together with a barrage of some 50 Tomahawk missiles, pounded targets in Syria. The US President said the presence of the five partners - Bahrain, Jordan, Saudi Arabia, Qatar and the United Arab Emirates - was a clear demonstration that this was not a US fight alone.
Sept 24, 2014: The United Nations Security Council unanimously approved a resolution cracking down on the flow of foreign fighters to terrorist groups at a session chaired by US President Barack Obama. At the same time, US air raids targeted ISIS oil installations in eastern Syria that have helped fund the group's brutal rise from rebel faction to alleged global threat.
Oct 3, 2014: ISIS releases a video showing the beheading of British aid worker Alan Henning. It threatens American aid worker Peter Kassig with the same fate.
Nov 16, 2014: A video of American aid worker Peter Kassig's beheading is released, making him the fifth Westerner whose execution is recorded and released by ISIS.
Jan 20, 2015: ISIS threatens to kill two Japanese hostages, Kenji Goto and Haruna Yukawa, unless a US$200 million ransom is paid.
Jan 24, 2015: A video of Japanese hostage Kenji Goto is released. In the video, Goto appeared to be holding a photograph depicting the execution of his fellow Japanese hostage Haruna Yukawa. A voice which is said to belong to Goto then asked for the release of an Al-Qaeda-linked attempted female suicide bomber, Sajida al-Rishawi, in exchange for his freedom.
Jan 26, 2015: ISIS is driven out of Kobane, Syria's third largest Kurdish town on the border with Turkey, after more than four months of fierce fighting led by Kurdish forces backed by coalition air strikes.
This ISIS failure comes on the same day an Iraqi official says the eastern province of Diyala has been freed of the extremist group.
March 28, 2015: Al-Qaeda's Syrian affiliate Al-Nusra Front and its allies seize Idlib, the second provincial capital lost by Syria's government.
May 17, 2015: ISIS fighters seize Ramadi, the capital of Anbar province in Iraq.
May 20, 2015: It gains control of the ancient city of Palmyra in Syria.
Source: Washington Post, BBC, Guardian, Institute for the Study of War, CNN