By Lee Ferran and Rym Momtaz
Born from an especially brutal al Qaeda faction, the Islamic State of Iraq and Syria (ISIS) has grown from relative obscurity in recent years to overshadow its extremist patrons. It now terrorizes large swaths of Syria and Iraq, has become the target of the largest U.S. military operation in Iraq in years and, with the public, cold-blooded execution of multiple Westerners, dominates headlines the world over.
Where Did ISIS Come From?
While extremist groups are generally amorphous organizations, ISIS can trace its history directly back to the Sunni terrorist organization al Qaeda, specifically the Iraq faction, al Qaeda in Iraq (AQI). AQI, led by Abu Musab al-Zarqawi, was responsible for scores of bombings, kidnappings and beheadings in Iraq following the U.S. invasion there. After al-Zarqawi was killed in 2006 by an American airstrike, leadership of the group eventually fell to an experienced Iraqi fighter, Abu Du’a, better known as Abu Bakr al-Baghdadi.

AQI was weakened in Iraq in 2007 as a result of what is known as the Sunni Awakening, when a large alliance of Iraqi Sunni tribes, supported by the U.S., fought against the jihadist group. AQI saw an opportunity to regain its power and expand its ranks in the Syrian conflict that started in 2011, moving into Syria from Iraq. By 2013, al-Baghdadi had spread his group’s influence back into Iraq and changed the group’s name to ISIS, “reflecting its greater regional ambitions,” according to the U.S. State Department. ISIS, as the group has been identified by ABC News and other news organizations, refers to the Islamic State of Iraq and Syria. Different translations of the Arabic name al-Baghdadi gave his organization have spawned other English-language versions such as the Islamic State of Iraq and al-Sham (also ISIS) or the Islamic State of Iraq and the Levant (ISIL).

READ: US Says 'No Reason to Doubt' Veracity of Video Featuring Shadowy Iraqi Terror Leader
Is ISIS Part of Al Qaeda?
Although originally an al Qaeda affiliate, ISIS and al-Baghdadi had a public falling out last year with Ayman al-Zawahiri, Osama bin Laden’s replacement and leader of al Qaeda “core,” over the role of another al Qaeda group, the al-Nursa Front, in Syria. This February a letter obtained by the Long War Journal reportedly showed al Qaeda’s senior leadership was so fed up with al-Baghdadi that it severed all connection with ISIS. By declaring himself the “caliph” of the Islamic State in June, al-Baghdadi appears to have challenged al-Zawahiri directly for the allegiance of all Muslim extremists.
How Has ISIS Been So Successful in Iraq?
ISIS has seen a series of successes as it has cut its way from Syria into Iraq and towards Baghdad using a combination of military expertise and unimaginable brutality. Social media accounts associated with ISIS have published disturbing videos purportedly showing ISIS fighters taunting, torturing and executing scores of unarmed prisoners. In addition, former senior U.S. military officials who served in Iraq and helped train the Iraqi security forces said that ISIS has been able to take advantage of government forces who lack the motivation to put up a good fight against ISIS in some areas.

The Iraqi government and much of its military officer corps are mostly made up of Shi’a Muslims, whereas much of the areas ISIS has retained in Iraq is predominantly Sunni, like ISIS –- meaning the Iraqi military forces are often operating in areas where the local population may be more willing to tolerate, or even support ISIS. ISIS has also built relations of convenience with disgruntled local Sunni tribes and ex-Baathists who have felt marginalized and disenfranchised by the Maliki-led government in Baghdad, which has been accused of favoring Shi’as. ISIS wasn’t handed its first major defeat until mid-August when Kurdish and Iraqi forces, supported by an aggressive U.S. aerial bombing campaign, pushed the terror group off the Mosul Dam, a key piece of infrastructure.

READ: Why Control of a Terrifying Dam in Iraq Is Life or Death for Half Million People
How Big Is ISIS and Where Are They Getting Their Fighters?
Western officials only have rough estimates on ISIS’s total fighting force, but the CIA says the group could have more than 30,000 fighters. Most disturbing to western security officials, they say, is the huge portion of foreign fighters who left their homes and at times traveled halfway around the world to join the terror group. Matthew Olsen, the director of the National Counterterrorism Center, said in July that more than 12,000 foreign fighters had traveled to Syria to fight with extremist groups, many of them now counted among ISIS’s ranks, including more than 1,000 westerners, around 100 of them American. Later estimates put the number of foreign fighters in Syria at approximately 15,000.

READ: Hundreds of Western Fighters in Middle East, Official Says
Is ISIS a Threat to the U.S.?
Though al-Baghdadi has threatened the U.S. in general before, ISIS has primarily focused its attention on its regional ambitions. That may have changed with the prominent U.S. role in pushing ISIS off the Mosul Dam, as evidenced in the video of the execution of American journalist James Foley and those that followed. In Foley’s video, a purported ISIS fighter addressed President Obama directly, saying Foley’s death is retaliation for the U.S. airstrikes. Likewise, ISIS threatened the U.K. to halt its support of strikes before murdering a British national. In mid-September, an ISIS spokesperson called on Muslims the world over to kill civilians in any country that joined the U.S.-led coalition against the terror group, including the U.S.

READ: 'Apparent' That James Foley Executioner Is British, Intel Agencies Race to ID Killer
ISIS Persecution of Minority Groups
ISIS has been especially ruthless in its targeting of minorities in areas that have come under its control. In addition to the scores who have been brutally murdered, countless Christians, Yazidis and other minorities have been forced to flee areas they’ve called home for centuries. In August, thousands of Yazidis had to be rescued by Kurdish forces after being trapped with very little food and water in harsh conditions on the Sinjar mountains where they hid from ISIS. ISIS also brutalizes fellow Sunni and Shi’a Muslims who do not ascribe to its extreme interpretation of the religion, and the group has destroyed priceless archaeological sites.

WATCH: Situation for Stranded Yazidis on Mount Sinjar Worsens
American journalist James Foley was on assignment for the news outlet GlobalPost to cover the conflict in Syria when he was abducted in November 2012. Foley’s family, U.S. officials and other reporters kept the kidnapping a secret until January 2013 when Foley’s family decided to break their silence and beg for their son’s return.
Foley’s captors, whoever they were, hadn’t made contact, the family said. The U.S. government increased its effort to find Foley, using unlikely messengers including the Turks, the Czechs and the Russians to get word to potential abductors on either side of the Syrian conflict. In November 2013 a ransom demand for an astronomical sum of money -- $132 million -- was received, according to GlobalPost CEO Phil Balboni, but it wasn't taken seriously. Efforts continued to free Foley, including a secret, failed U.S. military rescue mission in the summer of 2014. But following U.S. airstrikes on ISIS in August, Foley was killed on camera by a self-professed member of the terror group.

READ: Could Money Have Saved James Foley? ISIS 'Wasn't Serious' About Demands, Officials Say
Days after the video of James Foley’s execution emerged, similar footage appeared online, this time showing the apparent murder of another American journalist, Steven Sotloff. Sotloff was featured in Foley’s video, with the militant telling President Obama that Sotloff’s life was in the President’s hands. Undeterred, the U.S. continued to hit ISIS with dozens of airstrikes.

Sotloff was a freelance reporter whose work on the Middle East appeared in TIME and Foreign Policy among others. He disappeared in Syria in August 2013. After Foley’s execution video appeared online, Sotloff’s mother, Shirley, made a videotaped plea to ISIS leader al-Baghdadi to have mercy on her son.

At the end of the video that appeared to show Sotloff’s death, the militant in black stands beside another kneeling hostage, identified as a British citizen.

“We take this opportunity to warn those governments that enter this evil alliance of America against the Islamic State [ISIS] to back off and leave our people alone,” he says.