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, who had once been in U.S. custody in Iraq.

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). It is also known as Daesh, based on an Arabic acronym.

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Is ISIS Part of Al Qaeda?
Although originally an al Qaeda affiliate, ISIS and al-Baghdadi had a public falling out in 2013 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. In February 2014 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 2014, al-Baghdadi appears to have challenged al-Zawahiri directly for the allegiance of all Muslim extremists. Ever since jihadist organizations around the globe have taken sides.
How Has ISIS Been So Successful in Iraq?
ISIS saw 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 are 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 government in Baghdad, which has been accused of favoring Shi’as. ISIS wasn’t handed its first major defeat until mid-August 2014 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

The U.S.-led coalition against ISIS expanded its aggressive bombing campaign against the group into Syria in September 2014 and has bombarded the terror group virtually daily since. In February 2015, the U.S. military said that day by day, ISIS is “losing ground” in Iraq.
How Big Is ISIS and Where Are They Getting Their Fighters?
Western officials only have rough estimates on ISIS’s total fighting force, but in late 2014, the CIA said the group was believed to be up to 30,000 fighters strong including local supporters, and growing. 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.

Nicholas Rasmussen, the Director of the National Counterterrorism Center, told Congress in February 2015 some 20,000 foreign fighters from 90 countries had traveled to Syria to join one group or another -- 3,400 of those fighters are said to have come from Western nations, including over 150 from the U.S. “who have either traveled to the conflict zone, or attempted to do so.”

“It’s very difficult to be precise with these numbers because they come from a variety of sources that vary in quality,” Rasmussen said. “But the trend lines are clear and concerning.”
Is ISIS a Threat to the U.S.?
Though al-Baghdadi had threatened the U.S. in general before, ISIS primarily focused its attention on its regional ambitions prior to the U.S.-led bombing campaign. But as the U.S. and others continue to bombard ISIS targets, the group has repeatedly called on its followers in Western nations to conduct deadly attacks at home.

One of the gunmen in a dual terror attack in Paris in January 2015 claimed that he was part of ISIS, though the other shooters in that attack were linked to an al Qaeda affiliate. Days after the Paris incident, authorities in the U.S. announced they had arrested an Ohio man and ISIS supporter who planned to bomb the U.S. Capitol.

In addition to the so-called “self-radicalized” ISIS supporters, Western intelligence agencies are concerned about those who travel to Syria and Iraq to fight with ISIS before coming back home. “The battlefields in Iraq and Syria provide foreign fighters with combat experience, weapons and explosives training, and access to terrorist networks that may be planning attacks which target the West,” Rasmussen said in February 2015.
ISIS Persecution of Minority Groups, Women
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 2014, 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.

READ: How US Military Team Slipped On and Off Mt. Sinjar

In a particularly disturbing development, a pamphlet attributed to ISIS and shared online described the rules of dealing with female unbelievers as “slaves.” The rules allowed for forced intercourse, for instance, except in certain circumstances.
Death at ISIS’s Hands
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 and later became the first in a long, disturbing line of Western hostages killed apparently on camera by the terror group.

After Foley’s disappearance, his 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. Once again, the airstrikes continued and other Western hostages were killed on camera in similarly gruesome fashion.
Sotloff’s death was followed by the murders of British citizens David Haines and Alan Henning, and American Peter Kassig. In early 2015, ISIS released videos showing the apparent murders of two Japanese hostages, and a video of the group burning a Jordanian pilot alive.

ISIS’s last known American hostage, 26-year-old Kayla Mueller, died in the hands of the terror group, the White House said in February 2015, though it’s unclear how she died.
Mueller had traveled the world as an aid worker before her death, going wherever she thought she could help those in desperate need.
“For as long as I live, I will not let this suffering be normal,” she told her local newspaper in 2013. “[I will not let this be] something we just accept… It is important to stop and realize what we have, why we have it and how privileged we are. And from that place, start caring and get a lot done.”
[Last updated: Feb. 23, 2015]