Notes on Siege Warfare 1870-71

During their rapid advance into France the German field armies did not allow themselves to be delayed by French-held fortresses.They would pass them by, leaving detachments behind to either simply observe them or to isolate them such that the garrisons would not be able to operate against the German lines of communications. These detachments would generally be relieved eventually by reserve and Landwehr formations, and the question would arise of taking them.

Military logic required that no fortresses should be left in enemy hands in the rear. Almost all the French fortresses blocked vital rail routes, and the garrisons were a threat to German communications, in addition the forces observing them would invariably be needed elsewhere, thus the Grosses Hauptquartier decided that the fortresses were to be taken as and when the availability of siege artillery and other equipment permitted.


German Methods

One of three methods would be used to attack a fortress: the short Artillerieangriff ("artillery assault"), the formal attack (ie. a full siege), or the Bombardement (literally "bombardment" ). In all three cases the Belagerungsartillerie ("siege artillery") would play the dominant role. With the first two methods the objective was initially to silence the defending artillery thereby allowing the besiegers to approach close to the fortress. In the case of a Bombardement the object was to force the surrender of the fortress by destroying the town within the walls.

The method of attacking a fortress was in a process of change due to the introduction of rifled guns, which was in its infancy at the beginning of the war. Up until now the so-called Vauban Method had been employed when attacking a bastioned fortress with smoothbore guns. Using this the cannon fired at a range of at most 600 metres. The new rifled guns, which had much greater range, accuracy and effect required a different approach. The attack could begin from much greater distances, and their fire could reach all parts of the smaller fortresses. Targets could be hit and destroyed much more quickly. The new guns were generally set up 1500-2000 metres from the fortress, often even further away. The main defence line of each work facing the main direction of the assault would receive the attention of one battery of cannon. The main defensive works would be bombarded by mortar batteries, and the whole front attacked by flanking batteries, and others firing on them from the rear when possible.

The besieging batteries, all in carefully-selected positions, would face the defensive works in a wide circle and bring down a concentric fire which quickly destroyed the fortresses defensive potential and also wrecked the built-up areas to the rear. This was the Artillerieangriff, in which the batteries would be brought up close to a small fortress, and was used early in the war at Toul, Verdun, Soissons, Schlettstadt and Neufbreisach. All except Verdun fell quickly.

In the case of larger fortresses, such as Belfort, Strasbourg or Paris, the artillery had to deploy further away. Once the defensive fire had been sufficiently reduced the siege engineers could move closer to the fortress, then a second artillery position could be established from which the defending artillery could be destroyed. On some occasions as at Strasbourg the guns would need to be advanced again in order to attack works which could not be observed from further away.

In order to be able to mount an assault through the main wall a breach was required, in the past this was always made by guns firing from a short distance away across the main ditch. In 1870-71 this was done in three places at Strabourg by indirect long-distance fire, and once at Soissons by direct fire.

The Bombardement in its proper sense was used at Thionville, Montmedy, Mezieres, La Fere and Longwy. Although on each occasion full preparations were made for an Artillerieangriff the primary objective was to force the surrender of the fortress by destroying the built-up areas. The Bombardement method caused outrage in France and Great Britain during the war, being regarded as a brutal attack on civilians. The Germans defended it by claiming that it brought the matter to a speedy conclusion, thereby reducing suffering.


German Siege Artillery

In peacetime the German Festungsartillerie (fortress artillery, which was responsible for the equipping and fielding of siege trains) consisted of: Prussia- 85 Festungartilleriekompagnien (batteries) each of 100 men; the other German states had the following numbers of Festungsartilleriekompagnien: Bavaria- 16, Saxony- 4, Baden- 4, Württemberg- 4. On mobilisation the Prussian companies were divided in two and increased to 209 men. The other states (except Baden) increased the number of men to a similar level but did not divide the companies. Baden incresed the companies to 9 for war, with 100 men each. In August 1870 the full strength was:

Prussia- 164 kompagnien
Saxony- 4 kompagnien, the Prussian and Saxon companies had a combined strength of 34,600 men
Bavaria- 16 batteries with 3,800 men
Württemberg- 4 batteries with 900 men
Baden- 9 batteries with 900 men

This gave a total of 40,200 men

These troops were divided up among the various German fortresses with those near the western borders most heavily manned, since in the event of the war starting badly they would be first to face the threat of a siege.

In the years 1860-70 the transition from smoothbore to rifled artillery had progressed in Germany to the point where in terms of the siege artillery the smoothbore cannon had been replaced by 9cm, 12cm,long and short 15cm rifled guns. The smoothbore howitzers, mortars and Bombardementkanonen had been retained since rifled replacements for them were not yet available. During the war a small number of rifled 21cm mortars were constructed and used at several sieges. The following table details the most frequently-used guns:



Weight of Gun and Carriage

Weight of Shell

Maximum Range (Metres)

15cm Kanone

80,000 kg

27 kg


12cm Kanone

32,000 kg

15 kg


Short 15cm Kanone

54,500 kg

27 kg


Rifled 21cm Mortar

67,000 kg

80 kg


Smoothbore 23cm Mortar

22,500 kg

28 kg (Bombe)



Between 1865 and 1870 Prussia had built up a siege train consisting of: 200 rifled guns (60 x 5cm, 100 x 12cm, 40 x 9cm), 120 smoothbore guns and mortars. For each rifled gun there were 1,100, and for each smoothbore piece 500 or 600 shells available. This material, with powder, vehicles etc was unused and in excellent condition. Half of it wa stored at Magdeburg, the rest at Koblenz and Wesel on the Rhine. This siege train was regarded as sufficient for a siege of one major fortress.

On the outbreak of war the siege train was mobilised and used initially to besiege Strasbourg. Enough material was in the Prussian fortresses to form a second siege train, it consisted of 285 guns of all types, with a number of Festungsartilleriekompagnien being mobilised to man it. The second siege train was sent to Paris. In addition several smaller groups of siege guns were despatched. For the siege of Paris the Germans eventually assembled a total of 502 siege guns of all types. This was in addition to the material used to take the smaller French fortresses behind the German lines. Despite the huge effort there was a shortage of suitable guns and the smaller French fortresses had to be reduced one by one as guns became available. Much use was made of captured French guns, horses and transport. Eventually the North German Confederation was able to field a total of 931 siege guns of all types in France, with 1,116,555 shells provided for them.


Progress of a Typical Siege in 1870

Fortresses tended to fall very quickly in the face of attack by rifled guns. Siege batteries would be constructed quickly, ideally a battery would be completed during one night. Early in the morning a heavy and steady fire would commence, and eventually the defenders would begin to return the fire, firing at identified battery positions. The concentric fire from the besiegers would begin to take effect on the walls, breastworks and guns, and the houses behind the walls where numerous fires would break out. The defending fire would begin to slacken, with guns wrecked and gunners killed and injured, although for a time the defenders would be able to bring new guns into use until eventually these too would be obliged to stop firing. The battle might last for hours, one day or several days, with the besiegers bringing trenches closer to the walls and building new batteries closer to the fortress. Destruction in the town would increase, and the walls might become so damaged that guns could no longer be positioned on them. Eventually with guns wrecked, gunners killed, injured or exhausted, the walls badly damaged, heavy damage in the town, and the moral and physical resistance of the fortress broken, the white flag would be raised, first on a tower and then on a bastion, and a trumpet signal from the fortress would announce the approach of an emissary. Firing would now cease and negotiations soon lead to the end of the action. The besiegers would occupy the fortress on the following day. The German Festungsartilleriekompagnien would now be able to appreciate the effects of their fire at first hand, because they would have to take over the fortifications and material and make the place fully defensible as quickly as possible.

The French Fortresses and Fortress Artillery

In July 1870 the French had 60 batteries (batteries a pied) of fortress artillery each with 200 men. Most French fortresses had an insufficient number of guns. Like the Germans the French were in the transition from smoothbore to rifled guns, of which the French had 8cm, 12cm, and short and long 15cm types. Theses were slightly inferior to the German guns in accuracy.

The French fortresses were all built to the Vauban system and the original works had been strengthened over the years. The new rifled guns with their much longer range required a new approach to fortification in that works would need to be set up much further from the centre of the fortress, and in 1870 only Metz, Belfort and Paris had had such works built before the Franco-Prussian War. (Metz with the main French field army inside surrendered when faced with starvation, Paris was effectively being starved out when the war ended, and Belfort, with a garrison of 17,000 men and an energetic commander held out for 103 days after encirclement, although the Germans were only able to assemble the full means necessary to conduct a formal assault towards the end of the year and even then they had to detach forces to guard against possible French relief attempts. At Belfort some of the outlying works which posed the greatest problems for the Germans had been hurriedly built in August after the war had begun.)

Thus in 1870 the great majority of French fortresses counted as outdated opposite the new rifled artillery. On the other hand how much work can be carried out in a few years in a situation where no major sieges had occurred using rifled guns? The cost alone of updating all the fortresses would have been enormous (and during the late 1860s the French Government was constantly looking to reduce military spending). The state of the French fortresses was to some extent offset by the fact that the transition to rifled artillery was just beginning and the Germans had limited numbers of rifled siege guns available. The French learnt the lessons well and by 1914 there was a network of fortresses in Northern France with detached works (eg. Toul, Verdun, Epinal) which provided a far greater task for an attacker to reduce, even though the development of artillery had progressed further and guns were available that could smash the strongest works.

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© Martin Tomczak 2002