A siege is a prolonged military blockade and assault of a city or fortress with the intent of conquering by force or attrition. A siege occurs when an attacker encounters a city or fortress that refuses to surrender and cannot be easily taken by a frontal assault. Sieges usually involve surrounding the target and blocking the provision of supplies, typically coupled with artillery bombardment, sapping and mining to reduce fortifications.
Sieges probably predate the development of cities as large population centers. Ancient cities in the Middle East show archeological evidence of having had fortified city walls. During the Renaissance and the Early Modern period, siege warfare dominated the conduct of war in Europe. Leonardo da Vinci gained as much of his renown from the design of fortifications as from his artwork.
Medieval campaigns were generally designed around a succession of sieges. It was not until the Napoleonic era when dynamic striking columns forced armies into the field and the ever increasing use of, ever more powerful, cannon reduced the value of fortifications. In modern times, trenches replaced walls, and bunkers replaced castles. In the 20th century, the significance of the classical siege declined. With the advent of mobile warfare, one single fortified stronghold is no longer as decisive as it once was. While sieges do still occur, they are not as common as they once were due to changes in modes of battle, principally the ease by which huge volumes of destructive power can be directed onto a static target.
A siege can have four possible outcomes: The defenders can repulse it without aid from the outside, in which case the position is said to have been held; if the defenders prevail with outside aid the siege is deemed to have been relieved or raised. When a siege results in the attackers taking control of the besieged city or fortress but the defensive forces are able to escape, the outcome is characterized as evacuated, and if the attacking force emerges victorious and also destroys and/or captures the defenders, the besieged entity is reckoned as having fallen.
Ancient and medieval siege warfare
Ancient sources contain many stories of sieges, such as the siege of Jericho in the Old Testament or the Siege of Troy described by Homer in the Iliad.
Alexander the Great's Macedonian army was involved in many sieges. There are two which are of particular note: Tyre and Sogdian Rock. Tyre was a Phoenician island-city about 1km from the mainland, and thought to be impregnable. The Macedonians built a mole (causeway) out to the island. It is said to have been at least 60m wide. When the causeway was within artillery range of Tyre, Alexander brought up stone throwers and light catapults to bombard the city walls. The City fell to the Macedonians after a seven month siege. In complete contrast to Tyre, Sogdian Rock was captured by guile. The fortress was high up on cliffs. Alexander used commando like tactics to scale the cliffs and capture the high ground. The demoralised defenders surrendered.
An astonishing number and variety of sieges formed the core of Julius Caesar's conquest of Gaul. Julius Caesar in his Gallic Wars describes how at the Battle of Alesia the Roman legions created two huge fortified walls around the city. The inner circumvallation, 10 miles, held in Vercingetorix's forces, while the outer circumvallation kept relief from reaching them. The Romans held the ground in between the two walls. The besieged Gauls, facing starvation, eventually surrendered after their relief force defeat against Caesar's auxiliary cavalry.
The Sicarii Zealots who defended Masada in 74 were defeated by the Roman Legions who built a ramp 100 meters high up to the fortress's west wall.
The universal method for defending against siege is the use of fortifications, principally walls and ditches to supplement natural features. A sufficient supply of food and water is also important to defeat the simplest method of siege warfare: starvation. During a siege, a surrounding army would build earthworks (a line of circumvallation) to completely encircle their target, preventing food and water supplies from reaching the besieged city. If sufficiently desperate as the siege progressed, defenders and civilians might have been reduced to eating anything vaguely edible—horses, family pets, the leather from shoes, and even each other. On occasion, the defenders would drive 'surplus' civilians out to reduce the demands on stored food and water.
Disease was another effective siege weapon, although the attackers were often as vulnerable as the defenders.
slung about two projectiles per hour at medieval enemy positions.
To end a siege more rapidly various methods were developed in ancient and medieval times to counter fortifications, and a large variety of siege engines were developed for use by besieging armies. Ladders could be used to escalade over the defenses. Battering rams could be used to force through gates or walls, while catapults, ballistae, trebuchets, mangonels, and onagers could be used to launch projectiles in order to break down a city's fortifications and kill its defenders. A siege tower could also be used: a substantial structure built as high, or higher than the walls, it allowed the attackers to fire down upon the defenders and also advance troops to the wall with less danger than using ladders.
In addition to launching projectiles at the fortifications or defenders, it was also quite common to attempt to undermine the fortifications, causing them to collapse. This could be accomplished by digging a tunnel beneath the foundations of the walls, and then deliberately collapsing or exploding the tunnel. This process is known as sapping or mining. The defenders could dig counter-tunnels to cut into the attackers' works and collapse them prematurely.
Fire was often used as a weapon when dealing with wooden fortifications. In ancient Japan, where buildings used to store supplies were mostly wooden, a fire could drive opponents to starvation. The Byzantine Empire invented Greek fire, which contained additives that made it hard to put out. Combined with a primitive flamethrower, it proved an effective offensive and defensive weapon.
Advances in the prosecution of sieges in ancient and medieval times naturally encouraged the development of a variety of defensive counter-measures. In particular, medieval fortifications became progressively stronger—for example, the advent of the concentric castle from the period of the Crusades—and more dangerous to attackers—witness the increasing use of machicolations and murder-holes, as well the preparation of boiling oil, molten lead or hot sand. Arrow slits, sally ports (concealed doors) for sallies, and deep water wells were also integral means of resisting siege at this time. Particular attention would be paid to defending entrances, with gates protected by drawbridges, portcullises and barbicans. Moats and other water defenses, whether natural or augmented, were also vital to defenders.
In the European Middle Ages, virtually all large cities had city walls—Dubrovnik in Dalmatia is an impressive and well-preserved example—and more important cities had citadels, forts or castles. Great effort was expended to ensure a good water supply inside the city in case of siege. In some cases, long tunnels were constructed to carry water into the city. Complex systems of underground tunnels were used for storage and communications in medieval cities like Tįbor in Bohemia (similar to those used much later in Vietnam during the Vietnam War).
Until the invention of gunpowder-based weapons (and the resulting higher-velocity projectiles), the balance of power and logistics definitely favored the defender. With the invention of gunpowder, cannon and (in modern times) mortars and howitzers, the traditional methods of defense became less and less effective against a determined siege.
Mongol siege warfare
In the Middle Ages, the Mongol Empire's campaign against China by Genghis Khan and his army was extremely effective, allowing the Mongol hordes to sweep through large areas. Even if they could not enter some of the more well-fortified cities, they used innovative battle tactics to grab hold of the land and the people:
- "By concentrating on the field armies, the strongholds had to wait. Of course, smaller fortresses, or ones easily surprised, were taken as they came along. This had two effects. First, it cut off the principal city from communicating with other cities where they might expect aid. Secondly, refugees from these smaller cities would flee to the last stronghold. The reports from these cities and the streaming hordes of refugees not only reduced the morale of the inhabitants and garrison of the principle city, it also strained their resources. Food and water reserves were taxed by the sudden influx of refugees. Soon, what was once a formidable undertaking became easy. The Mongols were then free to lay siege without interference of the field army as it had been destroyed... At the siege of Aleppo, Hulegu used twenty catapults against the Bab al-Iraq (Gate of Iraq) alone. In Jūzjānī, there are several episodes in which the Mongols constructed hundreds of siege machines in order to surpass the number which the defending city possessed. While Jūzjānī surely exaggerated, the improbably high numbers which he used for both the Mongols and the defenders do give one a sense of the large numbers of machines used at a single siege." 1
Another Mongol tactic was to use catapults to launch corpses of plague victims into besieged cities. The disease-carrying fleas from the person's body would then infest the city, and the plague would spread allowing the city to be easily captured, although this transmission mechanism was not known at the time.
On the first night while sieging a city, the leader of the Mongol forces would lead from a white tent: if the city surrendered, all would be spared. On the second day, he would use a red tent: if the city surrendered, the men would all be killed, but the rest would be spared. On the third day, he would use a black tent: no quarter would be given.
This attitude was common to most armies. A city that surrendered could expect to negotiate terms to avoid a sack. A city broken by siege or assault could suffer extreme retribution, even in the 19th century.
Sieges in the age of gunpowder
The introduction of gunpowder and the use of cannons brought about a new age in siege warfare. Cannons were first used in the early 13th century, but did not become significant weapons for another 150 years or so. By the 16th century, they were an essential and regularized part of any campaigning army, or castle's defences.
The greatest advantage of cannons over other siege weapons was the ability to fire a heavier projectile, further, faster and more often than previous weapons. Thus, 'old fashioned' walls—that is high and, relatively, thin—were excellent targets and, over time, easily demolished. In 1453, the great walls of Constantinople were broken through in just six weeks by the 62 cannon of Mehmet II's army.
However, new fortifications, designed to withstand gunpowder weapons, were soon constructed throughout Europe. During the Renaissance and the Early Modern period, siege warfare continued to dominate the conduct of war in Europe.
Once siege guns were developed the techniques to assaulting a town or a fortress became well known and ritualised. The attacking army would surround a town. Then the town would be asked to surrender. If they did not comply the besieging army would surround the town with temporary fortifications to stop sallies from the stronghold or relief getting in. The attackers would then build a length of trenches parallel to the defences and just out of range of the defending artillery. They would then dig a trench towards the town in a zigzag pattern so that it could not be enfiladed by defending fire. Once within artillery range another parallel trench would be dug with gun emplacements. If necessary using the first artillery fire for cover this process would be repeated until guns were close enough to be laid accurately to make a breach in the fortifications. So that the forlorn hope and support troops could get close enough to exploit the breach more zigzag trenches could be dug even closer to the walls with more parallel trenches to protect and conceal the attacking troops. After each step in the process the besiegers would ask the besieged to surrender. If after the forlorn hope stormed the breach successfully the defenders could expect no mercy.
Emerging theories on improving fortifications
The castles that in earlier years had been formidable obstacles were easily breached by the new weapons. For example, in Spain, the newly equipped army of Ferdinand and Isabella was able to conquer Moorish strongholds in Granada in 1482–92 that had held out for centuries before the invention of cannons.
In the early 15th century, Italian architect Leon Battista Alberti wrote a treatise entitled De Re aedificatoria which theorized methods of building fortifications capable of withstanding the new guns. He proposed that walls be "built in uneven lines, like the teeth of a saw." He proposed star-shaped fortresses with low thick walls.
However, few rulers paid any attention to his theories. A few towns in Italy began building in the new style late in the 1480s, but it was only with the French invasion of the Italian peninsula in 1494–95 that the new fortifications were built on a large scale. Charles VIII invaded Italy with an army of 18,000 men and a horse-drawn siege-train . As a result he could defeat virtually any city or state, no matter how well defended. In a panic, military strategy was completely rethought throughout the Italian states of the time, with a strong emphasis on the new fortifications that could withstand a modern siege.
New styles of fortresses employed
The most effective way to protect walls against cannon fire proved to be depth (increasing the width of the defences) and angles (ensuring that attackers could only fire on walls at an oblique angle, not square on). Initially walls were lowered and backed, in front and behind, with earth. Towers were reformed into triangular bastions.
This design matured into the trace italienne. Star-shaped fortresses surrounding towns and even cities with outlying defenses proved very difficult to capture, even for a well equipped army. Fortresses built in this style throughout the 16th century did not become fully obsolete until the 19th century, and were still in use throughout World War I (though modified for 20th century warfare).
However, the cost of building such vast modern fortifications was incredibly high, and was often too much for individual cities to undertake. Many were bankrupted in the process of building them; others, such as Siena, spent so much money on fortifications that they were unable to maintain their armies properly, and so lost their wars anyway. Nonetheless, innumerable large and impressive fortresses were built throughout northern Italy in the first decades of the 16th century to resist repeated French invasions that became known as the Wars of Italy . Many stand to this day.
In the 1530s and 1540s, the new style of fortification began to spread out of Italy into the rest of Europe, particularly to France, the Netherlands, and Spain. Italian engineers were in enormous demand throughout Europe, especially in war-torn areas such as the Netherlands, which became dotted by towns encircled in modern fortifications. For many years, defensive and offensive tactics were well balanced leading to protracted and costly wars such as Europe had never known, involving more and more planning and government involvement.
The new fortresses ensured that war rarely extended beyond a series of sieges. Because the new fortresses could easily hold 10,000 men, an attacking army could not ignore a powerfully fortified position without serious risk of counterattack. As a result, virtually all towns had to be taken, and that was usually a long, drawn-out affair, potentially lasting from several months to years, while the members of the town were starved to death. Most battles in this period were between besieging armies and relief columns sent to rescue the besieged.
At the end of the 17th century, Marshal Vauban, a French military engineer, developed modern fortification to its pinnacle, refining siege warfare without fundamentally altering it: ditches would be dug; walls would be protected by glacis; and bastions would enfilade an attacker. He was also a master of planning sieges themselves. Before Vauban, sieges had been somewhat slapdash operations. Vauban refined besieging to a science with a methodical process that, if uninterrupted, would break even the strongest fortifications.
Examples of Vauban-style fortresses in North America include Fort McHenry in Baltimore, Maryland, Fort Ticonderoga in New York State, and La Citadelle in Quebec City.
Planning and maintaining a siege is just as difficult as fending one off. A besieging army must be prepared to repel both sorties from the besieged area and also any attack that may try to relieve the defenders. It was thus usual to construct lines of trenches and defenses facing in both directions. The outermost lines, known as the lines of contravallation, would surround the entire besieging army and protect it from attackers. This would be the first construction effort of a besieging army, built soon after a fortress or city had been invested. A line of circumvallation would also be constructed, facing in towards the besieged area, to protect against sorties by the defenders and to prevent the besieged from escaping.
The next line, which Vauban usually placed at about 600 meters from the target, would contain the main batteries of heavy cannons so that they could hit the target without being vulnerable themselves. Once this line was established, work crews would move forward creating another line at 250 meters. This line contained smaller guns. The final line would be constructed only 30 to 60 meters from the fortress. This line would contain the mortars and would act as a staging area for attack parties once the walls were breached. It would also be from there that sappers working to undermine the fortress would operate.
The trenches connecting the various lines of the besiegers could not be built perpendicular to the walls of the fortress, as the defenders would have a clear line of fire along the whole trench. Thus, these lines (known as saps) needed to be sharply jagged.
Another element of a fortress was the citadel. Usually a citadel was a "mini fortress" within the larger fortress, sometimes designed as a last bastion of defense, but more often as a means of protecting the garrison from potential revolt in the city. The citadel was used in wartime and peacetime to keep the residents of the city in line.
As in ages past, most sieges were decided with very little fighting between the opposing armies. An attacker's army was poorly served incurring the high casualties that a direct assault on a fortress would entail. Usually they would wait until supplies inside the fortifications were exhausted or disease had weakened the defenders to the point that they were willing to surrender. At the same time, diseases, especially typhus were a constant danger to the encamped armies outside the fortress, and often forced a premature retreat. Sieges were often won by the army that lasted the longest.
An important element of strategy for the besieging army was whether or not to allow the encamped city to surrender. Usually it was preferable to graciously allow a surrender, both to save on casualties, and to set an example for future defending cities. A city that was allowed to surrender with minimal loss of life was much better off than a city that held out for a long time and was brutally butchered at the end. Moreover, if an attacking army had a reputation of killing and pillaging regardless of a surrender, then other cities' defensive efforts would be redoubled.
Advent of mobile warfare
Siege warfare dominated in Western Europe for most of the seventeenth and eighteenth centuries. An entire campaign, or longer, could be used in a single siege (for example, Ostend in 1601–04; La Rochelle in 1627–28). This resulted in extremely elongated conflicts. The balance was that while siege warfare was extremely expensive and very slow, it was very successful—or, at least, more so than encounters in the field. Battles arose through clashes between besiegers and punative relieving armies, but the principle was a slow grinding victory by the greater economic power. The relatively rare attempts at forcing pitched battles (Gustavus Adolphus in 1630; the French against the Dutch in 1672 or 1688) were almost always expensive failures.
However, this pattern was eradicated by the French Revolutionary and Napoleonic Wars. New techniques stressed rapidly moving armies that would clash on the open field of battle, and one single fortified stronghold was no longer as decisive as it used to be. Advances in artillery made previously impregnable defenses useless. For example, the walls of Vienna that had held off the Turks in the mid-seventeenth century were no obstacle to Napoleon in the late eighteenth. Where sieges occurred, the attackers were usually able to defeat the defences within a matter of days or weeks, rather than weeks or months as previously. But Lines of Torres Vedras (1810–1811), which were built by the Portuguese under the direction of Royal Engineers of the British Army during the Peninsular war were able to stop a French Army and were the first example of Trench warfare. The Siege of Sevastopol (1854–1855) during the Crimean War and those of Petersburg (1864–1865) during the American Civil War showed that modern citidals could still resist an enemy for many months. This era of rapidly moving armies continued through the 19th century. For example, the great Swedish white-elephant fortress of Karlsborg was built in the tradition of Vauban and intended as a reserve capital for Sweden, but it was obsolete before it was completed in 1869.
Advances in firearms technology without the necessary advances in battlefield communications gradually led to the defence again gaining the ascendacy. During the Franco-Prussian War, the battlefield front-lines moved rapidly through France. However, the Siege of Metz and the Siege of Paris held up German armies for months at a time due to the superior firepower of the Chassepot rifle and the principle of detached or semi-detached forts with heavy-caliber artillery. This resulted in the construction of fortress works across Europe such as the massive fortifications at Verdun.
Mainly as a result of the increasing firepower (such as machine guns) available to defensive forces, First World War trench warfare briefly revived a form of siege warfare. Although siege warfare had moved out from an urban setting because city walls had become ineffective against modern weapons, trench warfare was nonetheless able to utilize many of the techniques of siege warfare in its prosecution (sapping, mining, barrage and, of course, attrition) but on a much larger scale and on a greatly extended front. The development of the armoured tank and improved infantry tactics at the end of World War I swung the pendulum back in favour of maneuver.
The Blitzkrieg of the Second World War truly showed that fixed fortifications are easily defeated by maneuver instead of frontal assault or long sieges. The great Maginot Line was bypassed and battles that would have taken weeks of siege could now be avoided with the careful application of air power (such as the German paratrooper capture of Fort Eben-Emael, Belgium, early in World War II). The most important 'sieges' of the Second World War were on the Eastern Front where bloody urban warfare marked the battles of Leningrad, Stalingrad and Berlin. In these battles, the ruins of an urban landscape proved to be just as effective an obstacle to an advancing army as any fortifications. In the west apart from the Battle of the Atlantic the sieges were not on the same scale as those on the European Eastern front; however, there were several notable or critical sieges: the island of Malta for which the population won the George Cross, Tobruk and Monte Cassino. In the South-East Asian Theatre there was the siege of Singapore and in the Burma Campaign sieges of Myitkyina, the Admin Box and the Battle of the Tennis Court which was the high water mark for the Japanese advance into India.
The air supply methods which were developed and used extensively in the Burma Campaign for supplying the Chindits and other units, including those in sieges such as Imphal, as well as flying the Hump into China, allowed the western powers to develop air lift expertise which would prove vital during the Cold War Berlin Blockade.
During the Vietnam War the battles of Dien Bien Phu (1954) and Khe Sanh (1968) possessed siege-like characteristics. In both cases, the Vietminh and Vietcong were able to cut off the opposing army by capturing the surrounding rugged terrain. At Dien Bien Phu, the French were unable to use air power to overcome the siege and were defeated. However, at Khe Sanh a mere 14 years later, advances in air power allowed the United States to withstand the siege. The resistance of US forces was assisted by the PAVN and PLAF forces' decision to use the Khe Sanh siege as strategic distraction to allow their mobile warfare offensive, the first Tet offensive to unfold securely. The Siege of Khe Sanh displays typical features of modern sieges, as the defender has greater capacity to withstand siege, the attacker's main aim is to bottle operational forces, or create a strategic distraction, rather than take a siege to conclusion.
Despite the overwhelming might of the modern state, siege tactics continue to be employed in police conflicts. This has been due to a number of factors, primarily risk to life, whether that of the police, the besieged, bystanders or hostages. Police make use of trained negotiators, psychologists and, if necessary, force, generally being able to rely on the support of their nation's armed forces if required.
One of the complications facing police in a siege involving hostages is the Stockholm Syndrome where sometimes hostages can develop a sympathetic rapport with their captors. If this helps keep them safe from harm this is considered to be a good thing, but there have been cases where hostages have tried to shield the captors during an assult or refused to co-operate with the authorites in bringing prosecutions.
The 1993 police siege on the Branch Davidian compound in Waco, Texas, lasted 51 days, an atypically long police siege. Unlike traditional military sieges, police sieges tend to last for hours or days rather than weeks, months or years.
In Britain if the siege involves perpetrators who are considered by the British Government to be terrorists, then if an assault is to take place, the civilian authorities hand command and control over to the military. The threat of such an action ended the Balcombe Street Siege in 1975 but the Iranian Embassy Siege in 1980 ended in a military assault and the death of all but one of the hostage takers.
- May, Timothy. "Mongol Arms." Explorations in Empire, Pre-Modern Imperialism Tutorial: the Mongols. University of Wisconsin-Madison. 27 June 2004.
- Duffy, Christopher. Fire & Stone: The Science of Fortress Warfare (1660–1860). 1975. 2nd ed. New York: Stackpole Books, 1996.
- Duffy, Christopher. Siege Warfare: Fortress in the Early Modern World, 1494–1660. Routledge and Kegan Paul, 1996.
- Duffy, Christopher. Siege Warfare, Volume II: The Fortress in the Age of Vauban and Frederick the Great. Routledge and Kegan Paul: London, 1985.
- Lynn, John A. The Wars of Louis XIV.