Modern Siege Warfare

The military campaign by the Syrian regime in Aleppo and by the U.S.-led coalition in Mosul reveal a strange new paradox of modern combat: the difficulty, if not impossibility, of reclaiming urban terrain from entrenched rebels or insurgents without paying a high humanitarian price. It is “strange” because at first blush, the offensive firepower of today’s armies would seem to work in their favor. Yet, even in the face of heavy artillery and indiscriminate air strikes, under-armed rebels have consistently been able to hold on to large swaths of cities. And civilians trapped in these rebel-held areas, sometimes against their own wishes, are the principal victims.

Syrian President Bashar al-Assad, for example, has now retaken 75 percent of the Rebel-held eastern part of Aleppo, but it took several years and Russia’s intervention to do so. The rebel's call for a ceasefire today, to allow for the evacuation of civilians, will either prolong the war, if honored, or cause extreme levels of civilian suffering, if not. (At this point, neither Assad nor Russia appear willing to negotiate a ceasefire.)

Siege warfare, of course, predates medieval times. It occurs when an invading army, unable to capture a castle or city outright, surrounds it as a way to starve one’s enemies into capitulation. The tactic is generally associated with conventional wars between countries of relatively equal stature, in which an adversary besieges a city with particular significance for its opponent in order to tangibly impact the military or government, as well as psychologically affect the population. Think Stalingrad or Warsaw during World War II. Siege warfare has also been used over time by rebel armies as a form of irregular warfare against an established government. Anyone who has seen Hamilton: An American Musical knows that George Washington’s ragtag Continental army effectively employed this strategy against the larger and better-trained British forces in Boston and Yorktown. Siege warfare was also used in other civil wars, including during the U.S. Civil War (Vicksburg), the Spanish Civil War (Madrid), and more recently in the Balkans conflict (Sarajevo)—all to mixed success. 

More recently, we’ve seen an uptick in siege warfare by nations against irregular or rebel forces, such as Russia’s counterinsurgency in 1999 in the Chechen capital of Grozny. Technological advances in warfare would appear to favor modern nations and their armies—precision bombing, better intelligence, surveillance, and reconnaissance capabilities, and so forth. And it would thus be logical to assume that sieges would appeal most to a militarily dominant nation-state fighting an apparently less capable rebel or insurgent group—but current events reveal the opposite. 

As we've seen in Syria, Assad’s approach to defeating the opposition has indeed relied time and time again on siege warfare which, combined with the manipulation of humanitarian aid, has led to a strategy of “siege and starve until submission.” The logic behind Assad’s approach is twofold. He is reportedly short of the manpower he needs to take and hold territory and has to rely more and more on the assistance of non-statutory local and foreign militias. 

Siege warfare has thus emerged as an apparently attractive asymmetric approach—it is an alternative when an aggressor does not have the comparative strength to sack a city outright. Sieges give counterinsurgents a low-cost way to stay on the offensive, while committing fewer resources. Siege warfare is also appealing for counterinsurgents, especially non-democracies unconcerned with “winning hearts and minds” but looking to avoid direct confrontation, keep casualty numbers low, and slowly bleed the enemy into submission.

The second strategic aim of Assad’s counterinsurgency campaigns has been to downright prevent the rise of alternative governance in Syria. Siege warfare, combined with sustained attacks against the civilian population and infrastructure—hospitals, schools, and markets—can either destroy the opposition’s capacity to govern or create a political alternative to the regime. Moreover, because of their deliberately slow pace, sieges tend not to attract the same unwanted international scrutiny as more lethal forms of warfare.

A siege can also create perverse incentives for the besieged. Sieges are not meant to entirely blockade or suffocate a town, city, or area. Even the Syrian regime, as menacing as it has been, has allowed a narrow humanitarian corridor, in some cases, to provide rebels in the east of Aleppo with a lifeline. (Russian and Serbian forces allowed similar corridors in Grozny and Sarajevo, respectively.) But such outlets generally result in freezing the conflict, rather than tilting it toward any decisive victory. Lines of control rarely shift much, and the battle becomes mostly an all-or-nothing campaign of attrition, not one of gaining ground or shifting momentum. This can allow insurgents the time and space to regroup and rearm. Under a siege, even though food and ammo may be in short supply, there are often pauses of sorts, which allow the weaker side to mobilize their forces and boost morale.

Indeed, even in the case of Aleppo, the static defenses between the city’s east and west took years and a disproportionate amount of force to begin to budge. Until recently, imprecise barrel bombs as well as Russian airstrikes had done little to dislodge the rebels in the east. Even the weaponization of aid, by promising ceasefires and humanitarian corridors in exchange for surrender, may be ineffective at bringing the rebels to their knees. In fact, the rebels may be calling for a ceasefire as a tactic to prolong, not end, the war.

Civilians in cities can weather severe hardships and remain holed up nearly indefinitely, even against their wishes. In Sarajevo, for example, a small core of Bosnian soldiers relied heavily on ordinary citizens to take up arms and protect the city. These ad hoc groups of citizen-soldiers organized around existing social structures. In some cases, depending on a city’s political economy, sieges are sustained via underground criminal networks. The scholar Peter Andreas, for example, has written that although most of Sarajevo’s residents suffered mightily throughout the three-year siege, some prospered from the black market economy.

According to the data on twentieth-century warfare that we’ve gathered, the average length of a siege is just under one year (roughly eight months), but the longer a siege drags on, the more it favors the side under siege. In more modern times, according to the data, siege warfare is less militarily effective, especially in cases of civil war or asymmetric conflict. In Syria, a number of towns and smaller cities fell to Assad’s forces through shorter sieges where the stakes were presumably lower, while larger urban centers such as Aleppo became the center of gravity that the regime has only been able to sway through massive reliance on external support.

So how, then, do sieges end? Here it is important to make a distinction between sieges against a state and sieges against rebel groups or insurgencies. In Sarajevo, for example, it was the Bosnian state that was under siege, and the battle ended with the power-sharing accord reached at Dayton, which was something of a win for the Serbians. It is hard to see how such a resolution could be applied in the case of ISIS or Syrian rebels, who never really “owned” Aleppo or Mosul but rather occupied each in a campaign of resistance. With narrow chances of a grand political bargain on the horizon, sieges in Syria have so far ended either with the opposition breaking the siege or with rebel submission, followed by the regime takeover of the previously besieged town or village. In many cases, this takeover has been followed by a strategy of depopulation of the formerly rebel-held urban centers.

In Iraq, ISIS’ approach to siege warfare has alternated resistance with strategic withdrawal. In Mosul, ISIS rebels have dug vast underground networks of tunnels to maintain some freedom of movement and to continue to function, much like the Sarajevo tunnel built by the Bosniaks two decades ago. Additionally, ISIS commanders have already demonstrated their readiness to carry out mass public executions to deter defectors or informants so that they can maintain enough control of the city to meet their objectives. ISIS’ commitment to its cause presents a more significant challenge than, say, physically retaking the city or the population’s own resistance. Yet, on other occasions, the group has proudly affirmed that it is ready to abandon towns or villages and to withdraw to “the desert”—an expression it uses to indicate a strategic withdrawal to the countryside.

The underlying logic of siege warfare is that localized wars of attrition can end by compelling one side to surrender important terrain. This logic breaks down when localized violence, no matter how extreme, has no impact on an insurgent’s calculations. Indeed, the lesson of modern siege warfare during counterinsurgency operations is that, like their conventional predecessors, entrenched rebels can withstand long assaults while maintaining their hold on the population centers they occupy. Sieges today can persist for a long time and even result in the utter devastation of the city before they end. When Russian tanks rolled into Grozny after its 1999–2000 siege, the United Nations called it “the most destroyed city on earth.” Aleppo may well hold that distinction today.

If nations continue to look to siege warfare as a popular approach to dislodge rebel or insurgent groups, as is apparently the case in Aleppo and Mosul, they should not expect these campaigns to be short-lived or, even, to achieve their goal.