Aleppo. Mosul. Sana’a. Mogadishu. Gaza. These war-ravaged cities are but a few examples of a growing trend in global conflict, where more and more of the world’s most violent conflicts are being fought in densely populated urban areas, at a tremendously high cost to the civilians living there. Despite their aversion to urban warfare, American and NATO military strategists increasingly acknowledge that the future of war is in cities. Concurrently, humanitarian agencies such as International Committee of the Red Cross (ICRC) and United Nations High Commissioner for Refugees (UNHCR) are adjusting their response to relief operations in urban centers in real time. This rise in urban violence and the resurgence of warfare in cities comes from three key factors: the global trend toward urbanization, increasingly volatile domestic political conditions in developing countries, and changes in the character of armed conflict.

In February 2018, the federal government of Brazil put the military in charge of all public security responsibilities in Rio de Janeiro—a decision that marked the first time the president has ordered the military to take control of a city since the country’s dictatorship fell in 1985. Throughout the past year, cases of violent deaths in Rio alone increased by 7.5%, rising from 6,262 in 2016 to 6,731 in 2017. This spike occurred despite the influx of over 10,000 additional soldiers and police forces sent to improve security in the city. This example is just one of many of the rising incidents of political violence and armed group activity leading to state forces being deployed for a broad range of urban operations, from counterinsurgency to high-intensity conflict.

The Brazilian military is also hardly the first one to deal with the immense difficulties of fighting in cities. In June 2014, the world watched in dismay as Iraqi Security Forces collapsed in the face of an Islamic State (IS) advance on Mosul, Iraq’s second largest city. After years of training by U.S. and coalition forces and millions of dollars invested in training and equipment, two Iraqi divisions—nearly 30,000 men—simply retreated to escape an assault by an insurgent force of roughly 800 militants. It would take over 100,000 Iraqi security forces and allied militias, with massive air support from the U.S.-led coalition, to reclaim the city and free the million remaining residents.

The apparent ease within which IS took over cities like Mosul and Tikrit in Iraq and Deir el-Zour across the border in Syria was also puzzling considering that historically, urban-based insurgencies have been relatively rare and largely unsuccessful. For instance, with the exception of South Yemen in 1967, the urban-warfare strategy has been ineffective in places like El Salvador, Guatemala, and Peru. Throughout the 1970s and the 1980s, the urban terrorism campaigns of separatist groups such as Northern Ireland’s Provisional Irish Republican Army (IRA) and Spain’s Euskadi Ta Askatasuna (ETA), as well as the left-wing and right-wing extremist groups in Italy, France, and Germany also ultimately failed to accomplish their goals.

This poor track record might not come as a surprise. Cities, after all, are the bastions of state power. And as the epicenter of political, industrial, economic, and commercial activity, communications, and culture, cities lie at the center of transportation networks, where the state can bring its full power to bear. Scholars of classical insurgencies and civil wars have therefore often argued that cities offer easier targets for state control than the pacification of large rural areas in the periphery of the country.

Improvements in military technology have also widened the gap between state armies and irregular groups, making it more difficult to organize and mobilize an urban-based uprising against the vastly superior government forces. Overall, as a RAND Corporation study of global insurgencies has found, “urban insurgencies have traditionally been the easiest kind to defeat.”

The Metamorphosis of Warfare: Urbanization, Political Instability, and the Rise of Non-State Actors

Despite this history of ultimately failed urban insurgencies, and in spite of the non-negligible challenges would-be insurgents face in cities, global political violence is not only on the rise, but it is also increasingly more urban than rural. From urban riots and endemic urban violence in India and Pakistan to the ISIS-sponsored and -inspired shootings, bombings, and vehicular terrorism attacks in Barcelona, Paris, Brussels, Istanbul, and other major European cities and groups like al-Shabab and Boko Haram carrying out large-scale attacks in cities in Kenya and Nigeria, armed groups, insurgents, terrorists, and criminal gangs are managing violence unseen in the past. As a whole, a growing proportion of the world’s most violent conflicts are fought in cities like Aleppo, Raqqa, Mosul, Marawi, Gaza, Mogadishu, Donetsk, Saana, and many others, where conventional state forces fight against armed groups that exploit the urban terrain to make up for their relative weakness.

Behind this rise in urban violence and the resurgence of warfare in cities is a confluence of global demographic trends, domestic political power dynamics, and changes in the character of armed conflict.

Most basically, violent conflicts happen where people live, and in the 21st century, for the first time in world history, more people live in urban areas than in rural ones. This change of global urbanization happened very rapidly. In 1990, the world population was 43% (2.3 billion) urban. By 2015, it had grown to 54% (4 billion). By 2050, nearly two-thirds of the global population will live in cities. And while urbanization and population growth are global trends, the world’s least developed countries, especially in sub-Saharan Africa and parts of Asia, are growing and urbanizing at a much faster rate than their wealthier counterparts.

Now, it is true that urbanization is generally a positive phenomenon because it contributes to industrial growth and poverty reduction. Yet, when urban governance, security, and public services do no keep pace with rapid population growth, it increases opportunities for non-state actors to compete politically and challenge the government militarily. The combination of urban poverty, high population density, dwindling resources, and poor governance therefore leaves cities such as Cairo, Lahore, Dhaka, and Kinshasa particularly vulnerable to political and social unrest, criminal violence, and terrorism.

When political violence surges and the local police and security forces are overwhelmed, military forces must restore order. This trend is a problematic one considering that the history of urban warfare mainly involved militaries’ fighting for cities, not in them. Despite the historical examples of major battles that occurred in urban environments such as Stalingrad, Hue, and Mogadishu, military theorists and doctrine have long advised militaries to avoid, bypass, or isolate cities rather than conduct planned operations in them.

Thus, as military forces—which are neither trained, nor organized, nor equipped to operate in urban settings—are increasingly tasked with restoring political stability in cities, we are witnessing catastrophic destruction and more civilian deaths than any other type of military action. This phenomenon is most apparent in the Middle East, where fighting in cities and towns across Iraq, Syria, and Yemen is creating what the ICRC has described as a “new scale of urban suffering . . . where no one and nothing is spared by the violence.”

In a rapidly urbanizing world, weak state structures are then a source of urban violence, as armed groups learn to exploit popular discontent and weak governance to establish their presence in urban areas. At the same time, this growing trend for armed groups to fight in cities capitalizes on the fact that the urban environment, within itself, obfuscates, if not in effect alters, the balance of power between conventional state forces and armed non-state groups.

Consider, for instance, that fighting in cities is extremely manpower intensive, both as a result of the high military casualty rates and the need to guard continuously virtually every building taken from enemy forces. But as the Russians discovered during the First Chechen War, more troops do not necessarily translate into effectiveness on the urban battlefield, let alone victory.

Cities also tend to negate the technological advantages of conventional forces by rendering aerial intelligence assets and advanced weaponry significantly less effective. As such, insurgents can avoid detection and maintain easy mobility while military forces are easily targeted and constrained in their pace and maneuvers. This dynamic helps explain how, despite being outnumbered and outgunned, armed groups manage to overcome local security forces capacities, forcing states to deploy military forces that destroy the very cities they are sent to save.

The Urban Future of Global Conflict

The future of global violence is urban. While rural insurgencies have not vanished, recent trends reflect the rise of intrastate conflict involving non-state actors using the advantages of cites to achieve their political goals. Moreover, as Craig Whiteside and Vera Mironova point out, the “industrial production and utilization of suicide bombers” during the Mosul campaign as well as the increasing use of weaponized drones highlight aspects of modern urban warfare that will likely characterize future urban operations.

In fact, leading voices within the U.S. defense community now acknowledge that future wars will likely be fought in densely populated cities and that the U.S. military will probably be involved in some capacity in any number of these conflicts. It is therefore critical for the U.S. and its allies and partners to reassess their preparedness for these eventualities.

First, and perhaps most urgently, the U.S. military should invest more resources in preparing its soldiers and units for urban terrain. The current approach is ad hoc and inefficient. There are no urban operations units, schools, or researchers in the U.S. military today. A single specialized urban army unit should be formed to explore the necessary manning, training, and equipping needs for operations within cities rather than today’s construct of all units being designed for war in the desert or rolling hills of Eastern Europe. The U.S. military has jungle, mountain, and arctic warfare schools, but not an urban one. A premier institute dedicated to the study of dense urban environments could provide the missing component of a realistic site to train military forces as well as experiment with new tactics and equipment, a dedicated cadre of urban experts, and a group of researchers committed to the holistic study of urban conflict.

At the same time, any such training, let alone the execution of military operations, must ensure that lethal force is used in accordance with the humanitarian and legal norms in armed conflict. This is a complex and deeply contested topic, especially given that autocratic governments such as those of Syria and Russia, and even the Saudi-led coalition in Yemen, have repeatedly used weapons and tactics that fail to distinguish between combatants and non-combatants, directly violating the laws of war. Yet, the complexity and density of urban terrain and the presence of large civilian populations pose a serious challenge even for the technologically advanced militaries of democratic states like the U.S., leading NATO countries, and Australia, which generally strive to comply with international humanitarian law.

Another complicating factor is that international treaties designed to protect civilians in times of war can have unintended consequences when it comes to fighting in cities. The international treaty banning wartime use of chemical weapons, for example, prohibits militaries from using tear gas, which is a nonlethal and relatively less destructive crowd control technique often utilized by domestic police forces. Military effectiveness and ethnics are not mutually exclusive. But these tensions highlight the need for a better understanding of the precise operational, political, and legal conditions that make different civilian protection measures more or less effective.

The recent military victories in Mosul and Raqqa led to the wholesale destruction of these cities. Clearly, there is a serious need to revise how militaries fight in dense urban areas. With greater dialogue and coordination between the military and humanitarian agencies, the United States and its allies can not only improve their knowledge of modern urban warfare, but also be better prepared for future wars in cities.