(CNN) -- Here's a look at Islamic State in Iraq and Syria (ISIS).
Facts: Started as an al Qaeda splinter group.
Also known as Islamic State in Iraq and the Levant and Islamic State (IS).
Led by Abu Bakr al-Baghdadi.
ISIS is known for killing dozens of people at a time and carrying out public executions, crucifixions and other acts. It has taken over large swaths of northern and western Iraq.
The group currently controls hundreds of square miles. It ignores international borders and has a presence from Syria's Mediterranean coast to south of Baghdad. It rules by Sharia law.
ISIS's initial strategy for revenue was through extortion and robbery. Recently, al-Baghdadi's strategy shifted to generating resources through large-scale attacks aimed at capturing and holding territory.
Timeline: 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 Shia community.
June 7, 2006 - Al-Zarqawi is killed in a U.S. 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 are killed in a joint U.S.-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 Islamic State in Iraq and the Levant (ISIS).
April 2013 - Al-Nusra Front leader Abu Mohammed al-Jawlani rejects ISIS's attempt to merge with the group.
February 3, 2014 - Al Qaeda renounces ties to ISIS after months of infighting between al-Nusra Front and ISIS.
May 2014 - ISIS kidnaps more than 140 Kurdish schoolboys in Syria, forcing them to take lessons in radical Islamic theology.
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 28, 2014 - Iraqi Kurdistan restricts border crossings into the region for refugees fleeing the fighting.
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).
June 30, 2014 - The United Nations announces that an estimated 1.2 million Iraqis have been forced from their homes.
June 30, 2014 - The Pentagon announces the United States is sending an additional 300 troops to Iraq, bringing the total U.S. forces in Iraq to nearly 800. Troops and military advisers sent to Iraq are there to add security to the U.S. Embassy and the airport in Baghdad, and to provide support to Iraqi security forces.
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.
August 8, 2014 - Two U.S. F/A-18 jet fighters bomb artillery of Sunni Islamic extremists in Iraq. President Barack Obama has authorized "targeted airstrikes" if needed to protect U.S. personnel from fighters with ISIS. The U.S. military also could use airstrikes to prevent what officials warn could be a genocide of minority groups by the ISIS fighters.
August 19, 2014 - In a video posted on YouTube, U.S. journalist James Foley, missing in Syria since 2012, is decapitated by ISIS militants. The militants then threaten the life of another captured U.S. journalist, believed to be Steven Sotloff.
September 2, 2014 - ISIS releases a video showing the beheading of U.S. journalist Steven Sotloff. Sotloff's apparent executioner speaks in what sounds like the same British accent as the man who purportedly killed Foley. He's 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.
September 11, 2014 - The CIA announces that the number of people fighting for ISIS may be more than three times the previous estimates. Analysts and U.S. officials initially estimated there were as many as 10,000 fighters, but now ISIS can "muster between 20,000 and 31,500 fighters across Iraq and Syria," a CIA spokesman tells CNN.
September 13, 2014 - ISIS militants post video on a website associated with the group, showing the apparent execution of British aid worker David Haines. This makes him the third Western captive to be killed by the Islamist extremist group in recent weeks. ISIS directs a statement at British Prime Minister David Cameron, threatening more destruction if Britian continues its "evil alliance with America." At the end of the video, the executioner threatens the life of Alan Henning, another British citizen held captive. The executioner appears to be the same one who killed both Steven Sotloff and James Foley.