The fortified position of Liège ( [PFL]) was established after the First World War by Belgium to block the traditional invasion corridor through Belgium between Germany and France. The Belgian experience of World War I, in which the Belgian Army held up the Germans for a week at Liège, delaying the German invasion of France, caused Belgium to consider a similar defensive strategy before World War II. Belgium rebuilt the Liège fortifications and extended them onto the Pays de Herve (Herve plateau) closer to Germany, using the most advanced fortification technology available.
The PFL was divided into the modern defensive line, anchored on the Albert Canal by Fort Eben-Emael and extending to the south through a planned five additional forts, designated PFL I and the ring of forts around Liège. Liège commanded crucial road and rail crossings of the Meuse and was as strategically important in the 1930s as in 1914. The modernized Liège forts were called PFL II.
In 1936, Belgium's neutrality was proclaimed again by King Leopold III of Belgium in a vain attempt to forestall another invasion, preventing France from being able to make use of the Belgian defences and territory in the forward defence of France. At the outbreak of the Second World War, Belgian defences had to resist alone until France could advance into Belgium after its neutrality had been violated. Again the fortifications could not hold against the Germans.
Background
When Belgium gained its independence from the Netherlands in 1830, it inherited four lines of Napoleonic forts called the Wellington Barrier that the state could not and did not want to maintain. The forts faced France, a nation that Belgium had fought alongside for its independence. The dismantling of the Wellington Barrier forts began in 1839, but after the Revolutions of 1848 and the establishment of the Second French Empire in 1851, Belgium faced the possibility of invasion by France. Belgian leaders sought a new defensive strategy and struggled with popular anti-military sentiment or opposing commercial interests regarding existing forts, but settled by 1859 on a plan championed by Belgian general Pierre Emmanuel Félix Chazal. Beginning in 1847, Chazal called for Antwerp to be fortified and expanded into the National Redoubt as a fallback stronghold for the Belgian Army. The task of constructing the National Redoubt was given to a protégé of Chazal, Captain Henri Alexis Brialmont. Brialmont completed the National Redoubt in 1868, but technological advancements in artillery and another change in the geopolitical landscape of Europe again rendered Belgium's defensive strategy inadequate.
The Franco-Prussian War of 1870–71 had enormous geopolitical consequences for Belgium. A now-united Germany in the German Empire had annexed Alsace-Lorraine from France and thus ensured another war between the two nations. The new path for French or German soldiers into each other's nations was through the lightly defended Meuse river valley – through lightly defended southern Belgium and the unfortified French border. According to Chazal's doctrine, the forts of the Meuse were to be the fulcrum and crossing points for the army at Antwerp. At that moment, those fortresses numbered two: the citadels of Liège and Namur, which together controlled 18 out of 26 crossings over the Meuse. Within Liège alone were seventeen roads with twelve bridges over the Meuse and three train stations linking seven rail lines. The heights near the city commanded not only the nearby Herve and Hesbaye plains, but a gap between Liège and the Dutch border. For over a decade following the Franco-Prussian War, several Belgian Ministers of War and even Otto von Bismarck urged the fortification of the Meuse. In 1882, Prime Minister Walthère Frère-Orban finally commissioned draft plans for fortresses to be built at Liège and Namur, and the strategically important crossings at Visé and Huy.
Brialmont was tasked with those drafts, but again political debate slowed the militarization of the Meuse until 31 December 1886, when Brialmont was invited to conduct another study. He finished and presented his report on 15 January 1887, calling for a system of military installations around Liège and Namur similar to the one he built around Antwerp. His plans were approved on 1 February 1887, but political squabbling delayed the passing of a budget for Brialmont until June 1887.
Design and construction
By the 1870s, technological advances in artillery such as rifling, melinite, and delayed fuzes, had given it greater range, accuracy, and destructive power. Bastion forts – the tradition in European military architecture for a century – had become obsolete. In response to artillery's increased reach, military architects began in the 1850s to build detached polygonal forts farther away from cities to protect them from bombardment. To counter its new destructive power, architects began making those forts more durable.
In 1887, French military engineer Henri-Louis-Philippe Mougin produced plans for a "fort of the future" (Fort de l'Avenir) that would be mostly underground, be built of concrete, and be armed with artillery in retractable steel gun turrets. Concrete was found to be much more resistant to new artillery shells and following its use in the upgrading of the Séré de Rivières system in 1887 it became the standard building material for fortresses. The success of ironclad warships in the Crimean War inspired the other innovations of Mougin's design: gun turrets and metal armour. The initial strides in this field were made by Germans Hermann Gruson and , but Frenchmen like Mougin himself perfected fort gun turrets and made them retractable using counterweights. The Swiss built a prototype of Mougin's fort , as did the French , near Verdun. Mougin's plan was never adopted by the French military, but Brialmont was likely familiar with it.
At Liège, 12 forts – six small and six large – were to be placed in a ring from the city. The circumference of the city's fortress ring was about , with a gap of around between some of the forts, held by intermediate works. The ring forts were placed away from ravines or depressions that could conceal attackers. There was also to be a fort at Visé to guard its ford over the Meuse, and at Huy, half-way between Liège and Namur. Construction of the Meuse forts was first estimated to cost 24 million francs, and this was the sum given to Brialmont on 1 June 1887 even before his geological surveys were complete. To his consternation, no funds were allotted for Visé or Huy, and Brialmont had to economize his plans for the forts he could build. His forts had two traces, a triangular or a trapezoidal shape, depending on the terrain, and their structures were similarly regimented. There were three plans for the gorges and two for the central bunker, its casemates, and connecting galleries. The Meuse fortresses would be the first to be built entirely of concrete. The 'base' of a fort faced Liège, as did its entry ramp. During the battle of Liège, the fortress ring was bolstered with the addition of 31 redoubts and 63 trenches, behind and in front of the forts and surrounded by barbed wire.
The Belgian government awarded the contract to build the forts on 1 July 1888 to the French firm Hallier, Letellier Brothers, and Jules Barratoux. Construction began on 28 July 1888 with the clearing and excavation of the sites and building of storage and work structures. Work was finished three years later, on 29 October 1891, and cost ₣71,600,000. Around Liège, workers excavated of earth, poured of concrete, and laid of brick.
Protection
Brialmont designed the Meuse forts to withstand the power of the heaviest artillery of his day: pieces whose shells put out of force. As a response to the 1886 melinite tests, he covered the masonry in the walls with a thick layer of concrete which itself was then covered with of soil. The mixtures of concrete used, of which there were two, were determined by testing at . Concrete was poured into wooden frames and left in place for over two weeks. Once dry, mortar would be brushed over the concrete until smoothed on the interior, and covered in soil on the exterior. The vulnerability of a structure determined the thickness of the concrete protecting it; the gorge wall of a barrack was thick, while the exposed top of the massif was thick. Protection against infantry was offered by a sea of barbed wire on the glacis around the ditch and the casemates of the gorge, housing 5.7 cm cannons and garrison infantry. These troops were also to be mustered in the massif for a sortie in case of a hostile infantry assault.
The German siege artillery that engaged the Meuse forts in World War I generated an excess of of force. Compounding the deficit between modern artillery and the Meuse forts was that concrete could not be poured at night because of a lack of illumination equipment. Left to dry overnight, concrete poured the day before would not fully bond with the concrete of the next day. The consequences of this were severe. The explosion that annihilated the Fort de Loncin in August 1914 dislocated many different layers of concrete, resulting in immense damage to the entire fort. The relative thinness of the gorge front concrete would allow the Belgian Army to, in the event of a fort's capture, shell it from the city—from behind. The concrete and armour around the windows would be thus eroded until shells were exploding inside the barracks, forcing the garrison deeper within. The Imperial German Army, by entering Liège and Namur while attacking their forts, were able to do exactly this and to devastating effect.
Armament
The Meuse forts were armed with a total of 171 gun turrets, whose manufacture, transportation, and installation cost ₣24,210,775 in total. They were made of steel and placed in a concrete well in the massif or main redoubt. A turret itself sat on top a set of rollers, allowing it to turn 360°, and moved up and down its well with tracks running the length of the well. While smaller turrets could completely retract into the fortress, larger-calibre guns could not because of the length of their barrels. A steel collar protected the lip of the turret well and to prevent the displacement of the turret in case the nearby concrete was damaged. In October 1912, however, a cycle of artillery tests held in Russia and attended by Belgian officials showed that collar to be ineffective. 15 cm shells could jam a turret in place, while 28 cm shells could entirely dislodge them.
The turret and the guns within were moved, aimed, and fired from within the turret. This was done either by an observer within the turret directly, or indirectly with the use of a graduated ring that showed 1/20ths of a degree. According to Brialmont's specifications, a gun turret should make a complete rotation in 1.5 minutes, and three rotations in five minutes. Once the right angle and elevation was found, a brake was engaged to hold the turret in place. Ordinance was hoisted in a basket with a manual elevator to the gun, which was fitted with a hydraulic recoil brake containing a solution that was 80% glycerine and 20% water. The turret was kept under positive pressure using a manual ventilator to expel fumes from the gun and to keep fumes from incoming shells out of the fort. In case of a malfunction, a gun could be removed and replaced within a few hours. Munitions were held beneath the turrets in separate form, and without cartridge casings for larger-calibre guns and in a complete shell for 5.7 cm guns. Every one of the Meuse forts' guns used black powder and were never modified to use a smokeless alternative.