In this Saturday, June 14, 2014 file photo Iraqi Shiite fighters deploy with their weapons in Basra, Iraq. (Nabil Al-Jurani/AP)

As you listen to the latest news about Islamic State in Iraq, you may have been puzzled by the prominence of Shiite militias, fighting alongside the Iraqi army against IS.

Throughout the region—and far beyond, from Colombia to Sudan—militias support governments in military actions against non-state armed groups in areas in which the state has been weak or effectively absent.

Why do so many civil wars feature such “third actors” “that operate alongside regular security forces or work independently of the state to shield the local population from insurgents”?

Here’s a recent answer from the leader of an Iraqi Shiite militia, fighting on the government side against IS: “It is a guerrilla war and needs special fighting groups and skills. No regular army can fight such a war.” Militias have not only been crucial in reoccupying major cities, but in the eyes of the leader of Asaib Ahl al-Haq, the League of the Righteous, have also better capabilities for holding territory.

While the U.S. still seeks to avoid collaborating with Iran-backed Shiite militias, the Iraqi government forges semi-official alliances with those that appear to be more loyal. Indeed, in this region and elsewhere, such “militias,” “paramilitaries,” or “civil defense forces,” often manage to re-establish some form of political stability with surprising success. The drop in violence in Iraq in 2007, for example, has been attributed to a combination of “the surge” and the U.S. collaboration with Sunni militias.

Beyond militias’ special capabilities, states have many reasons to collaborate with militias—but they rarely fully control what these armed groups are doing.

Here are five things recent research suggests you should know about militias:

1. Governments often take advantage of militias by strategically encouraging or restraining their use of violence against civilians, as shown for example by Jessica Stanton. The characteristics of such government-militia collusion are shaped by government incentives. Sabine Carey, Neil J. Mitchell, and Michael Colaresi argue that states have incentives to avoid accountability for the use of violence by forming and exploiting informal ties to militias, especially when they are weak democracies or receive financial aid from other democracies.

Conversely, Kristine Eck shows with regard to authoritarian states that they often—openly or covertly—support militias to collect sensitive information in civil wars. After military purges in particular, access to intelligence is difficult, and so locally recruited militias may offer a valuable alternative source of information.

2. Not all governments collude with militias because of purely military incentives, however. Paul Staniland demonstrates that states intentionally sponsor those militias that support its ideological project. Pakistan, for example, has long colluded with Islamist armed groups, while India has developed various relationships with tribal and ethnolinguistic militias in the country’s northeast.

These arguments help to explain why the collaboration between the Iraqi government and the Shia militias is much more complicated than a purely military logic in the fight against the IS would imply.

3. Militias aren’t simply submissive agents fully controlled by the state. The fragmentation of territorial control between militias and government forces in recently re-occupied cities in Iraq is a counter-example to one of the main misconceptions about militias: that they are submissive agents fully controlled by the state, a critique also developed by Paul Staniland.

Quite the contrary; militia organizations, their alliances with the state, and their behavior in civil war vary to a great extent. We distinguish in particular between those militias formed or directed by the state and those mobilized by communities themselves, independent of state forces.

While state-initiated militias can become more independent from the state’s influence over time, those with community origins may be co-opted by the state for its own purposes at later stages.

Moreover, even when states form or collaborate with militias for counterinsurgency purposes, militias do not necessarily behave as intended and supporting them can backfire in the long run. Colombia, where many former paramilitary fighters now enforce the ranks of the bandes criminales, is just one example.

4. How militias recruit and socialize their members has an enormous effect on how they behave—as is true for any other armed group in civil wars. Just like insurgent leaders, militia commanders have incentives (but often limited capacities) to screen potential members for “high-quality” recruits—those who are likely to play by the rules and stay committed to the group’s goals. Triggering defection from rebel groups is one potential mobilization strategy of militia leaders, but we still know little about which insurgents are most likely to defect.

Ben Oppenheim, Abbey Steele, Juan Vargas and Michael Weintraub provide evidence from Colombia that combatants who joined the insurgents for ideological reasons are less likely to side-switch and join paramilitaries than economically-motivated combatants, although indoctrination mitigates the latter’s propensity for defection.

Instead of (or in addition to) inducing insurgent defection, many militia leaders exploit social networks and local information in their recruitment strategies. Based on field research in Sierra Leone, Jonathan Forney shows that if social networks that provide information about potential recruits break down, militias may lose their ability to screen recruits carefully, enlisting fighters hard to discipline in their use of violence—with devastating consequences for the local population.

For Shiite militias in Iraq that use social media to attract foreign fighters, this argument implies they could have tremendous problems to discipline their forces in the future.

5. Almost every report about militias includes a story about their brutal tactics. In fact, militias frequently increase levels of violence against civilians. Often, however, militias’ use of violence against civilians is neither the result of governments delegating violence nor the result of opportunistic individuals. Dara Cohen and Ragnhild Nordås find that child soldiers are particularly likely to perpetrate sexual violence, a finding they argue is driven by the fact that militia leaders often use that type of violence to increase cohesion among these members.

They also show that militias trained by state armed forces are more likely to be involved in sexual violence. The latter finding supports the notion that sexual violence is often perpetrated as a practice (as opposed to being ordered or the result of pure opportunism), an argument advanced by Elisabeth Wood.

The extent and repertoire of violence is also dependent on the social context in which militias operate. Jessica Stanton argues, for example, that violence against civilians is likely to be lower when militia members recruit from the same ethnic group as the insurgents, as they are reluctant to target their own social constituency. These arguments raise cause for concern about the use of violence by Shiite militias, as their recruits become younger and the sectarian divide is getting deeper.

So what?

What can scholars and policy-makers learn from these findings? First, states have a wide variety of reasons to create or coopt militias, reasons we are just beginning to understand. Second, militias are not always mere auxiliaries of governments, as they can emerge, evolve, and wield violence independently of the state. Third, rather than treating civil wars as a two-actor game and militias as a residual category, studying all types of armed groups alongside each other is the most promising approach to understanding militias.

Militias act much as other armed actors do. They evolve in reaction to and interaction with them. They may even turn into challengers of the state. The changing alliances among various armed actors in Iraq demonstrate that yesterday’s partners may become today’s enemies and vice versa.

Several studies discussed in this post are published in the most recent special issue of the Journal of Conflict Resolution, “Militias in Civil War.” Sage is granting free access to the special issue until November 1. 

Corinna Jentzsch is assistant professor of international relations at the Institute of Political Science at Leiden University. Livia I. Schubiger is assistant professor in comparative politics at the London School of Economics and Political Science (department of government). Stathis N. Kalyvas is Arnold Wolfers Professor in Political Science and the Director of the Program on Order, Conflict, and Violence at Yale University.