Within a period of four years, Islamic State (IS) has risen from near defeat in 2010 to become one of the most successful modern insurgencies. At its height in 2014, it controlled 40 percent of Iraq. Three years later, soon to be eradicated from its last Iraqi territory in Mosul, the threat is far from over. Local insurgent groups, which fell into decline under IS, could resurface in the post-IS vacuum, al-Qaeda could rebuild itself in Iraq and IS itself will likely return to its roots as a non-territorial guerrilla insurgency.

Iraq’s Insurgent Groups

The Iraqi militant landscape is composed of a complex network of movements that may reemerge after IS’ eradication from Mosul. The origins of these groups sometimes pre-date the 2003 U.S.-led invasion. The Islamic Movement of Kurdistan (IMK), for example, was founded in the 1980s by the Muslim Brotherhood’s Sheik Uthman Abdul Aziz, while Ansar al-Islam (AI) — formed in 2001 by IMK splinter groups — incorporated Sunni Arabs and establish ties to al-Qaeda.

Following the U.S.-led invasion, a number of Sunni militant groups emerged largely for the purpose of opposing the occupation: Jaysh Rijal al-Tariq al-Naqshbandia; the 1920s Revolution Brigades (1920s RB), named after the historic Iraqi revolt against occupying British forces; the Islamic Army in Iraq (IAI); the Mujahideen Army (MA); Ansar al-Sunna Sharia (AS Shariah); and Hamas Iraq. Abu Musab al-Zarqawi’s Jamaat al-Tawhid waal-Jihad, which would later formally affiliate with al-Qaeda and eventually become IS, also took root. With the departure of U.S. troops from Iraq in 2011, these militant Sunni groups did not diminish but instead reoriented their focus to challenge the government of Prime Minister Nouri al-Maliki and oppose Iranian influence in Iraq.

Many Sunni insurgent groups denounced IS’ targeting and exploitation of Iraqis and even fought against the group. Both the Jihad and Reform Front (RJF) — which was formed with MA, IAI, AI and AS Sharia — and the Political Council for the Iraqi Resistance (IRPC) — which was formed with Hamas in Iraq and parts of the Islamic Army in Iraq, the Mujahideen Army, AS Shariah, the Fatiheen Army and the Islamic Front for the Iraqi Resistance (JAAMI) — were established in opposition to IS (Terrorism Monitor, December 3, 2008). Others, such as the 1920s RB and JRTN, denounced IS’ targeting of Iraqi civilians but nevertheless cooperated with the group.

IS’ unprecedented expansion and territorial gains from 2011 to 2014 generated extensive resources and attracted fighters. As a result, it overshadowed Iraq’s other Sunni militant groups, absorbing or defeating competing organizations and forcing others into hiding. As IS’ star now wanes, these groups may reconstitute themselves. Some could also offer a less extreme alternative to IS in Sunni areas, taking up the mantle of challenging Iranian influence, Shia militia groups and the perceived political marginalization of Sunnis.

JRTN, for example, was the second largest insurgent group in Iraq at its peak. It provided organization, funds and intelligence to other militant groups, including al-Qaeda in Iraq, the 1920s RB and AI, to carry out attacks. It also formed the General Military Council for Iraqi Revolutionaries (GMCIR) with the 1920s RB and other groups (Terrorism Monitor, June 26, 2014).  Although JRTN condemned IS’ targeting of Iraqis, it still provided critical support to IS in its capture of Fallujah and Mosul in 2014.  After Mosul, IS and JRTN fell out, and IS eventually forced JRTN into hiding. In late 2016, however, JRTN issued a call to resist IS in Mosul, suggesting it may now be plotting a return to a more prominent position (Niqash, November 1, 2016).

Guerrilla Tactics

Having lost its territorial hold in Iraq, it is likely that IS will return to its roots as a non-territorial guerrilla insurgency. This tactical shift has been seen in numerous recent attacks in Iraq, including a series of nighttime attacks in Tikrit in which 34 people were killed (al-Jazeera, April 5). These offensives are designed in part to divert resources from the battle in Mosul, but also signal a shift toward guerrilla tactics. A top Kurdish counterterrorism and intelligence official also warned that IS will probably move out of cities into deserts and mountains, likely using the Hamrim Mountains as a base of operations (Iraqi News, February 16, 2016).  Hundreds of IS fighters have also reportedly relocated from Mosul to the Makhoul Mountains near Baiji (Niqash, March 15). According to Iraqi MP Zaher Taher, intelligence reports show that over 1,100 IS fighters arrived in Diyala province, a likely location for an IS guerrilla front (Iraqi News, April 11).

Territorial losses incurred by IS have created space for al-Qaeda which, in line with other militant groups in Iraq, may be planning a comeback. Like many Iraqi groups, al-Qaeda disagreed with IS’ targeting of Iraqis (favoring “external” enemies, such as the United States, over Iraqis) even when the two groups were affiliated. As such, it may seek alliances with local insurgencies, such as JRTN or other groups that may either emerge or re-assert themselves. Although al-Qaeda and IS have been adversaries since their formal split in 2014, it is possible that they may again cooperate in the wake of IS’ territorial losses. Iraqi Vice President Ayad Allawi has indicated that these kinds of discussions are already underway between al-Qaeda and IS (ARA News, April 18).

Drivers of Extremism Remain

The rise of IS was due predominantly to drivers that cultivated the political terrain for a Sunni resistance. The dismantling of IS’ supposed caliphate in Iraq has done little to remove these roots of insurrection; instead, they may have become more pronounced. Former Prime Minister Maliki, whose divisive government stoked Sunni upheaval, was nominated in April as a candidate to for the presidency of the Iraqi National Alliance, Iraq’s largest political party, and could re-ascend to the premiership  (Iraqi News, April 14).

Iranian influence in Iraqi politics remains strong, and Shia-dominated Popular Mobilization Front (PMF) militias (Hashd al-Shaabi) now exceed over 100,000 fighters, some of which are directed by Iran and responsible for human rights violations against Sunnis. In addition, large parts of Sunni, as well as Kurdish, areas of Iraq have been destroyed and over three million Iraqis have been displaced, creating turmoil that may further kindle extremism.

As IS’ territorial dominance in Iraq comes to an end, the future of Sunni militant groups largely depends on the extent to which Iraq’s political and security systems integrate Sunnis and other groups, or decentralize control to provinces and regions. If this is not accomplished, the next wave of guerrilla warfare will be determined by the ability of militant Sunni groups to reconstitute and expand their networks and the capacity of Iraq’s security forces to engage in counterinsurgency operations.