The US Army’s Mosul Study Group released a report—What the Battle of Mosul Teaches the Force—in September 2017, a mere month and a half after the Islamic State was ejected from the city. The report purported to provide lessons derived from the battle. However, the short time that passed between the end of the battle and the report’s publication raises questions about the report and the group’s research methods. Moreover, the “intense 45-day effort,” as the authors described their work, to study the battle and produce the report yielded a view of the battle isolated from its place within the campaign and dislocated from trends in modern war. In doing so, the report inadvertently misleads the reader on particular lessons that Mosul illuminates.

The Mosul Study Group’s report is an important work, one whose contents should be read and understood. But it also either overlooked key aspects of the battle, did not fully develop certain lessons, or did not have sufficient time to appreciate the meaning of some conclusions. First, for example, the battle of Mosul was decisive in the classical military sense of the word—it triggered a fundamental military and political change in the war. However, if a decisive battle is not quickly followed with action to preserve the victory, or if the battle is fought in such a way that exceeds the winning side’s capability of preserving the victory, then a battle’s decisiveness can be squandered. Second, the battle of Mosul was a grinding positional siege that manifested itself as an urban, layered defense against a slow, methodical siege. Third, this positional siege was an example of the continued trend of transactional proxy warfare, in which the US Army largely outsources ground combat to a surrogate, while providing logistical support, fire support, and combat advising to the proxy. And fourth, the battle illuminated what might be called a Precision Paradox, a situation in which the promise of precision strike’s comparatively limited damage was counteracted by the consequent need to employ vastly greater numbers of strikes. This slowed Iraqi security forces’ tempo, thereby increasing the death and destruction in the city as the Iraqis methodically moved through Mosul. An additional byproduct of the over-reliance on precision strike was that it nearly depleted the stock of US precision munitions.

Before investigating these points in detail a brief survey of the battle is useful to provide context for the discussion.

The Battle of Mosul: A Brief Overview

The battle of Mosul was the centerpiece of the combined Iraqi and US-led coalition’s campaign to defeat the Islamic State in Iraq. A slow but successful campaign fought in Anbar province, which included a devastating battle in Ramadi, allowed Iraq and the coalition to shift their attention to the Islamic State stronghold in Mosul in the fall of 2016.

The battle of Mosul began on October 16, 2016. It officially concluded on July 10, 2017; however, the battle simmered on for several more weeks as Iraqi forces and their coalition counterparts mopped up residual holdouts within the city. Fighting in Mosul did not cease until August 2017.

Estimates vary, but the Islamic State fielded an army in Mosul that numbered somewhere between five thousand and twelve thousand soldiers. Conversely, the government of Iraq, in conjunction with the Kurdish Regional Government, commanded a combined force of approximately ninety-four thousand uniformed soldiers, along with an additional fourteen thousand sanctioned partisans.

For the Islamic State, meeting Iraqi forces and their coalition partners in open warfare in Iraq’s western deserts was a non-starter; doing so would have ensured quick destruction. Mosul, the Islamic State’s de facto Iraqi capital, was an excellent place for the group to plan for an encounter because the urban area offered the outmatched Islamic State an opportunity to offset its military shortcomings through a positional defense. This enabled the group to multiply the cost of war for the Iraqis and coalition to the point that it had its best chance of winning the battle. As a result, the Islamic State massed its army in Mosul and prepared its defenses.

In hindsight it is easy to argue that the Islamic State’s defeat was assured; however, the battle was not decided before it was joined. Drones, robots, and precision strike did not save the day. Brute force, willpower, and a combined industrial base that exceeded that of the Islamic State won the battle. Along the way, the Islamic State pushed the Iraqi forces to the brink of its culminating point—the point at which it was unable to continue its attack due to the attrition of its force, resources, and will to fight—while nearly depleting American precision-strike stockpiles. Further, the battle’s duration and destructiveness suggest a fight whose outcome was more tenuous than retrospective renderings represent.

Brute Force and Will Power: The Return of Decisive Battle

The Mosul Study Group didn’t sufficiently emphasize that the battle of Mosul was the “decisive battle” in the campaign to defeat the Islamic State; in fact, it never mentions that phrase. Not making this important fact clear is likely a byproduct of the speed with which the group’s report was published—they did not have the time to assess the battle’s effect on the campaign. The report does mention “decisive action.” In part, this is indicative of a broader problem related to Army doctrinal definitions. But it also means that the Mosul Study Group missed an opportunity to identify a trend increasingly characterizing war: the return of decisive battle.

US Army doctrine defines decisive action as “the continuous, simultaneous execution of offensive, defensive, and stability operations or defense support of civil authority tasks.” The definition is wide-ranging and devoid of explicit connection to any particular aspect of warfighting. More importantly, it misuses the word “decisive” itself. The dictionary definition of “decisive” is straightforward: “having the power to decide.” The Army definition fails to tie decisive action to the power to decide.

Theorists and historians offer better definitions of military decisiveness. In his masterpiece, The Ghost of Napoleon, B.H. Liddell Hart, seeking to flesh out the notion of decisiveness, states that an event only turns out to be decisive when its conditions allow one to gain a decision there. Historian Cathal Nolan, looking more at the macro-level, argues that decision in war is achieved when a battle, operation, or campaign achieves a useful strategic or political goal, which in turn results in the long-term maintenance of one’s respective interests.

In The Foundations of the Science of War, British theorist and military officer J.F.C. Fuller takes the discussion on decisiveness to the appropriate threshold. Fuller offers that decisiveness in war, whether at the policy or tactical level, is the result of cumulative action against an adversary that forces it to unwillingly accept the policy in dispute or to change its plan, the resultant effect being that the adversary then acquiesces because it sees the futility in any further active or overt resistance. In most cases, time, above all else, reveals a battle’s decisiveness.

Taking a step back from Mosul’s tactical confines, one sees that the battle packed lasting, decisive, results. As planned, the battle of Mosul was just one stop in a larger campaign that had begun somewhat haphazardly in Anbar province in the summer of 2014, and in a more deliberate fashion with the establishment of the Combined Joint Task Force – Operation Inherent Resolve in October of that year. Following Mosul, the campaign plan continued, with subsequent battles in deliberately selected places like Tal A’far, Hawijah, and along the Middle Euphrates River Valley. However, Mosul fundamentally altered the subsequent campaign in two notable ways.

First, the battle shattered the Islamic State’s army. It was unable and unwilling to put a significant army in the field following Mosul and hence transitioned to a strategy of evasion. The battle of Tal A’far illustrated this transition. Planned by Iraqi and coalition forces to begin in early August 2017 and match Mosul in intensity, the battle proved insignificant because the Islamic State did not stand and fight in Tal A’far. Instead, it offered cursory resistance and avoided being brought to battle. In fact, the Islamic State’s army never again massed for battle.

Second, the battle accelerated the divergence of interests between the government of Iraq and the US-led coalition. Once the Islamic State’s defeat was apparent, the government of Iraq began to distance itself from the coalition. The Iraqi government’s expedition to quell growing rumblings for Kurdish independence in October 2017, in spite of US opposition, is a prime example of how the battle influenced the partners’ policy objectives. Additionally, Iraqi Prime Minister Haider al-Abadi’s push to decrease the US troop presence in Iraq following the Islamic State’s defeat is another example of the battle’s effect on not only the campaign, but the overall war. In effect, Mosul’s decisiveness triggered a change in each belligerent’s plans, both militarily and politically.

More broadly, it is important to note Mosul’s decisiveness because the battle is an example of the increasing frequency of decisive battles in modern war. Looking beyond Iraq, the war in eastern Ukraine’s battle of Ilovaisk in August 2014 proved decisive because it birthed the Minsk Protocol between the governments of Russia and Ukraine, resulting in a temporary ceasefire between belligerents.

Later, the battles for Donetsk Airport and the siege of Debal’tseve were decisive battles that drove the Minsk II agreement and resultant stalemate in eastern Ukraine. Looking elsewhere, the siege of Marawi, a five-month positional battle of attrition pitted Filipino defense forces—with US military support—against the Islamic State in the Philippines. This battle also proved decisive in the campaign against Islamic State forces in that country.

The return of decisive battle is important to note because it contradicts some of the Information Age–concepts so regularly bandied about today and suggests that “likes” and “1s and 0s” are not the key to winning the next major war. Concepts such as those are important, to be sure, but principally because they help shape the environment and information space. They are auxiliary features in modern positional battles of attrition rather than potentially decisive capabilities. Instead, Mosul shows that positional battles of attrition—the ability to muster and replenish human capital, resources, and the will to fight at a rate greater than one’s opponent—will continue to decide the course of campaigns, wars, and policy for the foreseeable future.

Defensive Battles and Campaigns: Positional Feats of Attrition

The battle of Mosul was a positional battle of attrition in which both sides—the Islamic State’s army and the Iraqi and coalition forces—sought to physically exhaust and morally disintegrate the other. The US Army fought the campaign through a series of proxy forces, to include the Iraqi army, Kurdish security forces, and various militias from across the country.

The goal of positional warfare is to gain an advantage on the battlefield or rebalance disparities in combat power or resources. Essentially, the goal is to place oneself in a position of advantage relative to one’s opponent or to lure a belligerent into vacating its own position of advantage. This is accomplished by the application of force through tactics, movement, or firepower to relocate an opponent from one site or physical situation to another for further exploitation.

The Islamic State, likely understanding their weakness in terms of relative combat power, massed in Mosul. Based upon this decision, one can assume that the Islamic State’s purpose was to bring a degree of parity to the battle by attempting to positionally and functionally dislocate Iraqi and coalition forces and increase their investment costs. In doing so, the Islamic State nudged the scale in their operational and tactical favor. Although the group was ultimately defeated in Mosul, the resultant effect of its attritional defensive was a nine-month battle that nearly broke the Iraqi forces, pulverized Mosul, and came worryingly close to exhausting the coalition’s supply of precision weapons and other supporting equipment.

Mosul is one example within the larger campaign to regionally defeat the Islamic State. On the Syrian side of that campaign, a number of similar positional battles of attrition are apparent. Specifically, the sieges of Aleppo, Raqqa, Kobani, Deir ez-Zor, and Ghouta reinforce this point. Stepping beyond the battlefields of the Middle East, positional battles of attrition loom large in the war in eastern Ukraine. The battles of Donetsk Airport, Luhansk Airport, and Debal’tseve are the most striking examples.

The point is that while the Mosul Study Group made mention of layered defense within a dense urban environment, they did not articulate what that really means—that Mosul was a positional battle of attrition in which one combatant sought to push the other to the culminating point, while working to forestall its own culminating point in the process. This is important to note because, as current conflicts illustrate, the occurrence of positional battles of attrition are on the rise. This trend in war is likely to continue into the foreseeable future because the conditions that drive it—namely, a weaker belligerent seeking parity while the aggressor feeds proxy forces into the battle on its behalf—are not likely to dissipate anytime soon. In addition, so long as the positional battle of attrition’s layered urban defense continues, so too will its counterpoint, the siege.

More importantly, Mosul demonstrates that how an army wants to fight is not nearly as important as how an army will have to fight. While doctrine describes how a force wants to fight, many factors—from policy and national caveats to terrain and the enemy’s location—dictate how force will have to fight. With positional battles of attrition on the rise, the US Army must understand that phenomenon and be prepared to survive, fight, and win under such conditions. Therefore, as we look to the future, we should expect to see a rise in the urban defense, characterized by a positional contest of attrition in which victory is not guaranteed simply by having the best-trained soldiers or most high-tech equipment, but by being most able to muster the resources and resolve to weather the attritional slog.

The discussion of decisive battles is not complete without a mention of squandering victory. The effects of a decisive battle can be squandered if not followed by actions that preserve its military and political consequence. In the case of Iraq, for example, the Islamic State is slowly creeping back into previous strongholds—the result of insufficient and ineffective constabulary forces, inadequate reconstruction effort, and misbalanced representation of various factions in the government.

The manner in which the campaign was waged has also contributed to the conditions driving the Islamic State’s re-emergence. For example, the Iraqi Counter Terrorism Service suffered up to 60 percent causalities in Mosul, and Iraqi security forces as a whole incurred over ten thousand casualties in the battle. For the most part, Mosul’s attritional siege left the government of Iraq insufficient land forces to police newly liberated territory, thereby indirectly supporting the conditions for the Islamic State’s re-emergence.

Shia militia groups—collectively referred to as Popular Mobilization Forces—also played an undeniable role in the campaign against the Islamic State and subsequent pacification efforts. Although the degree of these combatant groups’ connections to Iran varies and is difficult to assess, a large number of them are Iranian proxies, which has created problems for the pacification effort. While their involvement in the campaign was tacitly supported, their continued employment fans the flames of sectarianism that facilitated the Islamic State’s rise in the first place, as well as increasing tension between the United States and Iran. In hindsight, the manner in which the Islamic State was fought has contributed to its re-emergence. Therefore, prudence dictates that future proxy wars be fought with an eye on the resultant peace, because as Liddell Hart argues, “The object in war is to attain a better peace—even if only from your own point of view. Hence it is essential to conduct war with constant regard to the peace you desire.”

Proxy Wars: Today’s Dominant Form of War

The Mosul Study Group did not stress the significance of the US Army’s application of proxy warfare during the battle of Mosul. Instead the group’s report spoke euphemistically about the Army’s transactional proxy relationships with Iraqi and Kurdish forces, defining them as “security force assistance” conducted through “advising, assisting, accompanying, and enabling.” The report would have been better served by elaborating the character of the principal–agent relationship that existed between the Army and its proxies in the campaign. This shortfall perpetuates the inability to understand the dynamics of proxy relationships, both one’s own and those of an opponent, and how those relationships changes in relation to mission accomplishment.

Further, in recent years proxy-fought battles have revealed distinctive characteristics. Since the summer of 2014, proxy battles supported by great powers or regional powers have tended to result in an attritional urban defense for the weaker opponent and a siege—the urban defense’s reciprocal response—by the stronger opponent. Yet, the US Army continues to advocate maneuver warfare and precision strike as the preferred method of engagement in decisive action, while omitting any real discussion of proxy warfare.

The problem with this is that, as Mosul illustrates, modern proxy wars—today’s dominant form of war—are more often than not positional battles of attrition. In turn, the US Army should invest time, intellectual energy, and resources to develop a theory of proxy war. As previously noted, a range of variables, not solely doctrine, drive how the Army fights. In many instances today, these constraints result in the US Army fighting with minimal land forces, in an advise-and-assist role, and as the principal actor in a principal-proxy relationship.

Further, while causality is not clear at this point, it is not a stretch to suggest that the rise in proxy wars has also resulted in the increase of positional battles of attrition because a principal actor is less concerned about the destruction of proxy forces in bloody ground combat than it would be about the destruction of its own army. Therefore, so long as great and regional powers continue to pursue interests in competition with one another and are simultaneously unwilling to expend their own human capital in the pursuit thereof, proxy wars will continue to dominate armed conflict.

The Precision Paradox

The Mosul Study Group lauded the US and coalition employment of precision strike during the battle for its ability to strike a given target and reduce the collateral damage. The problem with this assessment is that, while true on its face, it presents a clear fallacy when one steps back from an isolated individual strike and looks at the cumulative effect of those weapons over the course of the battle. The Mosul Study Group indirectly eludes to this by stating:

In Mosul, the destruction of physical terrain did not necessarily equate to comparable effects against personnel or communication nodes. Munition choices in Mosul, amplified by the structural density of the city, were not always proportional to the intended effects on the enemy and, when combined with rules of engagement considerations, on collateral damage. Even when considering overpressure and blast waves from these rounds, ISIS fighters were forced from their defensive positions by shrapnel or direct-fire weapon systems, rather than blast effects.

In many instances, when a target was struck a number of individuals subsequently fled from the building. Seeing those Islamic State fighters abscond into another set of houses or buildings resulted in additional precision strikes being dialed up and launched, with the effect repeated. If the initial strike did not kill every Islamic State fighter in the target location, then each target struck created multiple subsequent targets as fighters fled to other locations. In essence, this spidering effect from a proximal target location created a targeting loop that involved more strikes and thus more risk of collateral damage.

Aside from that, the ubiquitous threat of precision strike also drove Islamic State fighters underground. They would tunnel from house to house in order to avoid detection by overhead surveillance tools. In turn, this second-order effect from precision strike further increased destruction across the city.

This cause-and-effect loop, or Precision Paradox, was a situation in which the failed promise of precision strike—one strike, one kill—generated a creeping wave of destruction across the city. Reports vary, but around ten thousand civilians were killed during the battle, of which 3,200 were from coalition airstrikes and indirect fire. Of those civilians killed, many of the deaths were the result of being crushed to death in buildings either directly targeted by coalition strikes or adjacent to those targeted by the coalition. The city requires upwards of $2 billion to rebuild. The battle for western Mosul alone destroyed an estimated forty thousand homes and left ten million tons of detritus for the victors to deal with.

Additionally, the allure of precision strike and its associated rolling wave of destruction was one of the primary factors that attributed to the grinding positional battle of attrition that devastated the city. The sense of security provided by precision strike resulted in Iraqi land forces waiting to advance until coalition strikes shaped the target area in front of them. This caused the battle, especially in western Mosul, to evolve into a methodical grind as Iraqi forces would attempt to eliminate Islamic State fighters or positions sequentially before moving forward and maintaining offensive momentum.

It is instructive to also note that the over-reliance on precision strike severely cut into the American strategic stockpile of precision weapons. Open-source reporting toward the end of the battle highlights the fact that the US military was running out of Hellfire missiles and a number of other precision munitions during the campaign to defeat the Islamic State. Given the central role of precision weapons in the battle, this could have been quite problematic if the battle had lasted much longer than it did. This Precision Paradox is important for the US Army to understand as the service ponders future war.

By producing a report so rapidly, the Mosul Study Group overlooked a number of key conclusions that can be drawn from the battle of Mosul. Among these is the return of decisive battle, to include those against nonstate actors. While often overlooked, it is important to note that the battle of Mosul, which lasted nine months and four days, was only twenty-three days shorter than World War I’s longest battle, the battle of Verdun.

Further, a large urbanized area levels the playing field for an otherwise overmatched defending force. The duration and ferocity of Mosul illustrates this point—the Islamic State was able to drag Iraq and its seventy-three coalition partners into a grinding, nine-month, attritional affair that left Mosul ruined and Iraqi forces teetering on the brink of its culminating point, when it would no longer be able to conduct operations due to the exhaustion of personnel, resources, and the will to fight. Given the power and capability disparity between the belligerents, nothing but a well-fought defense could have achieved this result. One must assume that other potential adversaries have taken note, so it is prudent to expect that wars will increasingly feature defensive struggles waged in and around urban areas. Prussian strategist Carl von Clausewitz reminds the student of war that the defense is the stronger form of war and that its use is most often driven by one’s weakness and need for self-preservation. Moreover, as urban defenses rise, so too will its counterpoint, the siege.

Additionally, the battle of Mosul and the campaign to defeat the Islamic State are but individual data points in a wider set that clearly illustrates the growing trend toward proxy war. Contemporary proxy war is a function of two sets of circumstances. The first includes those situations in which defeating a heavily ensconced enemy requires great human cost. The second are those instances in which great powers or regional powers come into conflict with one another, but do so through indirect means.

The US Army must focus on developing a theory of proxy war, because how one must fight is more important than how one wants to fight. The Army should incorporate an examination of how best to bring nonstate actors to decisive battle. Army research should also focus on how to overcome a heavily entrenched enemy that is able to muster the resources, human capital, and the will to drag an opponent into a long, bloody struggle—while also keeping its attention oriented on the post-war situation and the goal of creating a situation in which enduring peace is achievable. Concepts that do not account for Verdun-esque positional battles of attrition are not aligned with the current tone of modern war, nor are those that omit or euphemistically sidestep the role and character of proxies in modern war. Further research into proxy war, specifically focused on understanding the relationship between principals and agents, is important and will help the Army better utilize its own proxy relationships, while finding effective ways to split hostile proxies from their hosts.

In addition, the US Army should launch empirical research on the Precision Paradox to further explore its details. The Army must account for the Precision Paradox and incorporate it into campaign and tactical planning doctrine. Doing so will allow Army leaders to more accurately weigh the costs of a precision strike–fueled proxy campaign against one in which more land forces are employed, thereby reducing collateral damage for the nation in which the war is waged.

Lastly, the Mosul Study Group report is an important and useful document, but because it was published amazingly quickly after the battle of Mosul, it also has significant limitations. Alongside that report, the Army should also ensure it takes a longer-term approach to gaining a true understanding of the battle’s real effect. Researchers should be afforded the time to allow the full nuances of the impact of battles like Mosul to truly be illuminated. Otherwise, the Army runs the risk of telling itself what it wants to hear and not what it needs to hear.


Amos C. Fox is a major in the United States Army. He is an armor officer currently stationed at Fort Carson, Colorado. He is a graduate from the US Army’s School of Advanced Military Studies, Ball State University, and Indiana University-Purdue University at Indianapolis. His previous assignments include tours with the 1st Armored Division, the 4th Infantry Division, the 11th Armored Cavalry Regiment, and the US Army Armor School. The views expressed are those of the author and do not reflect the official position of the United States Military Academy, Department of the Army, or Department of Defense.
Image credit: Staff Sgt. Jason Hull, US Army