When the US withdrew its last troops from Iraq in 2011, Al Qaeda in Iraq had been defeated and one of the Middle East’s most troubled countries had the chance to carve out a normal existence for itself.
Three years later it had all fallen apart.
Al Qaeda in Iraq turned into ISIS, seized Iraq’s second-largest city, took over large sections of Iraq and neighbouring Syria, and triggered a full-on regional crisis.
Between the divisively sectarian policies of former Iraqi Prime Minister Nuri al-Maliki and the chaos in neighbouring Syria, al-Qaeda in Iraq quickly went from a broken organisation operating at the fringes of society to a major military threat.
To make sense of the meteoric rise of ISIS, Frontline, PBS’s award-winning news documentary series, produced an in-depth look at the group’s ascendancy. Here’s what they found about the rise of a jihadist group that has become one of the most consequential players in the Middle East.
The rise of ISIS in Iraq would not have been possible without the sectarian policies of former Iraqi Prime Minister Maliki, a Shi'ite who used strong-armed tactics to rule a highly diverse country.
As soon as the US completed the withdrawal of troops from Iraq, Maliki attempted to arrest his Sunni Vice President, Tariq al-Hashimi. Hashimi's security guards were reportedly tortured until they confessed to charges of terrorism on video.
Hashimi fled Iraq, where he was convicted in absentia and sentenced to death. Meanwhile, Maliki continued to arrest thousands of Sunnis on dubious charges. Shia militias also carried out extrajudicial executions of Sunnis throughout Baghdad.
Maliki also enraged Sunni tribal officials who had previously fought against al-Qaeda in western Iraq by not paying their salaries and by stripping them of positions in the military and police force.
In early 2012, al-Qaeda in Iraq (the forerunner of ISIS) was essentially a broken organisation. Defeated during the Sunni Awakening and the US troop surge, it operated in the periphery of the country, near the border with a quickly imploding Syria.
The lack of central authority, along with the uprising against Syrian dictator Bashar al-Assad, allowed ISIS to make serious inroads in the collapsing country.
Within 12 months, ISIS had spread throughout Syria. It became an effective fighting force due to the group's previous experience fighting in Iraq -- and it got funding from citizens in living in the Gulf States.
But ISIS soon no longer needed funding from the Gulf as the group received more and more cash from extortion, ransoms, checkpoint controls, and illicit oil sales.
At the same time, Maliki continued purges of Sunni officials from the Iraqi government, leading to the well-respected Sunni finance minister, Rafi Issawi, joining in mounting protests in Anbar province.
These Sunni protests against Maliki's increasingly sectarian policies quickly spread throughout Iraq to other Sunni cities, including Mosul.
ISIS took advantage of the public outrage in Iraq against Maliki and his imprisonment of Sunnis. They launched what they called the 'Breaking the Walls' campaign, which was a series of strikes against Iraqi prisons. A number of the prisoners joined ISIS, swelling its ranks.
By March 2013, ISIS flags began to appear amongst protesters in Ramadi, signifying a shift from some Sunnis hoping for political reform to believing jihadism offered one way to address their grievances.
The turning point of the protests occurred in April 2013. A policeman was killed during clashes between Iraqi security forces and protestors. This led to a crackdown that resulted in the deaths of hundreds of Sunni civilians.
Then, in December 2013, Maliki made a fatal mistake. The prime minister sent the army to dismantle the protest camps in Ramadi and arrest Ahmed al-Awani, a Sunni politician. During the raid, al-Awani was taken by the government and his brother was killed.
Against a backdrop of intense anger, ISIS infiltrated Anbar and convinced tribal leaders and Sunni protestors that they were the only group that could protect them from the Shia-dominated government.
The resulting street battles in Fallujah between disenfranchised Sunnis and ISIS fighters pitted against the Shia-dominated military led to the city breaking away from government control.
In the spring and summer of 2014, Kurdish intelligence officials began to warn Maliki repeatedly that ISIS had encamped themselves in Mosul and was planning a strike against Iraq's second city. Maliki ignored them.
On June 6, 2014, ISIS began an assault on Mosul. Although the attack was only intended to be a prison break, the lack of effective resistance led to a force of 800 ISIS fighters and their Baathist supporters taking Iraq's second-largest city by June 10.
The fall of Mosul allowed ISIS to plunder an unprecedented amount of US military supplies, originally given to the Iraqi Security Forces. This weaponry allowed ISIS to continue fighting south along the Tigris River through other Sunni-dominated areas.
On June 29, capitalising on its battlefield success, ISIS did something no other jihadist group had ever done before. It declared a caliphate with its leader Baghdadi as caliph.
Also unlike previous jihadist organisations, ISIS has drawn a number of former Iraqi military officers into its ranks who want the overthrow of the Shiite-dominated government in Baghdad. These officers give ISIS a greater military capability than past groups.
ISIS's success in Syria and the lack of order in Iraq has allowed the group to depend on a flow of fighters and supplies between the two countries.
ISIS' ability to operate with relative impunity in Syria, combined with the local anti-government discontent among Sunnis in Iraq, has allowed ISIS to take over and operate across the two countries in an area approximately the size of Belgium.
A possible turning point for ISIS may have occurred when the group decided to attack beyond Sunni Arab areas and push into Kurdish territory in northern Iraq.
Although ISIS was initially successful, and sent streams of refugees flooding into Kurdistan, the threat to US interests in the region -- like the danger of ISIS taking over Iraqi Kurdistan -- drew the Americans into the war.
The US also began airstrikes against ISIS in Syria, although the overall effectiveness of American strategy has been widely questioned.
At the same time, US and Iraqi officials have started making overtures to Sunni tribal leaders in the hope of drawing them away from ISIS. However, they have proven unwilling of switching sides without their political grievances being fully addressed.
Until the political demands of Sunnis are met throughout Iraq, ISIS will continue to cement its authority and make gains in Sunni-dominated areas in Iraq.
The longer it takes to develop an effective strategy against ISIS, the more the group will embed itself in Syria and Iraq. Already, the jihadists are almost within artillery range of the Baghdad airport. The crisis that this once-defeated group has created is far from over.
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