The Battle of Liège (French: Bataille de Liège) was the opening engagement of the German invasion of Belgium and the first battle of World War I. The attack on Liège city began on 5 August 1914 and lasted until 16 August, when the last fort surrendered. The length of the siege of Liège may have delayed the German invasion of France by 4–5 days. Railways needed by the German armies in eastern Belgium were closed for the duration of the siege and German troops did not appear in strength before Namur until 20 August.
Belgian military planning was based on an assumption that other powers would eject an invader. The likelihood of a German invasion did not lead to France and Britain being seen as allies or for the Belgian government to intend to do more than protect its independence. The Anglo-French Entente (1904) had led the Belgians to perceive that the British attitude to Belgium had changed and that it was seen as a British protectorate. A General Staff was formed in 1910 but the Chef d'État-Major Général de l'Armée (Chief of the General Staff), Lieutenant-Général Harry Jungbluth was retired on 30 June 1912 and not replaced until May 1914 by Lieutenant-General Chevalier de Selliers de Moranville who began planning for the concentration of the army and met railway officials on 29 July. Belgian troops were to be massed in central Belgium, in front of the National redoubt of Belgium ready to face any border, while the Fortified Position of Liège and Fortified Position of Namur were left to secure the frontiers. On mobilisation, the King became Commander-in-Chief and chose where the army was to concentrate. Amid the disruption of the new rearmament plan, the disorganised and poorly trained Belgian soldiers would benefit from a central position, to delay contact with an invader but it would also need fortifications for defence, which were on the frontier. A school of thought wanted a return to a frontier deployment in line with French theories of the offensive. Belgian plans became a compromise in which the field army concentrated behind the Gete river with two divisions forward at Liège and Namur.
German strategy had given priority to offensive operations against France and a defensive posture against Russia since 1891. German planning was determined by numerical inferiority, the speed of mobilisation and concentration and the effect of the vast increase of the power of modern weapons. Frontal attacks were expected to be costly and protracted, leading to limited success, particularly after the French and Russians modernised their fortifications on the frontiers with Germany. Alfred von Schlieffen, Chief of the Imperial German General Staff (Oberste Heeresleitung, 'OHL') from 1891–1906, devised a plan to evade the French frontier fortifications with an offensive on the northern flank, which would have a local numerical superiority and obtain rapidly a decisive victory. By 1898–1899, such a manoeuvre was intended to pass swiftly through Belgium, between Antwerp and Namur and threaten Paris from the north. Helmuth von Moltke the Younger succeeded Schlieffen in 1906 and was less certain that the French would conform to German assumptions. Moltke adapted the deployment and concentration plan, to accommodate an attack in the centre or an enveloping attack from both flanks as variants, by adding divisions to the left flank opposite the French frontier, from the c. 1,700,000 men which were expected to be mobilised in the Westheer (western army). The main German force would still advance through Belgium to attack southwards into France, the French armies would be enveloped on their left and pressed back over the Meuse, Aisne, Somme, Oise, Marne and Seine rivers, unable to withdraw into central France. The French would either be annihilated by the manoeuvre from the north or it would create conditions for victory in the centre or in Lorraine on the common border.
At midnight on 31 July – 1 August, the German government sent an ultimatum to Russia and announced a state of "Kriegsgefahr" during the day; the Turkish government ordered mobilisation and the London Stock Exchange closed. On 1 August the British government ordered the mobilisation of the navy, the German government ordered general mobilisation and declared war on Russia. Hostilities commenced on the Polish frontier, the French government ordered general mobilisation and next day the German government sent an ultimatum to Belgium, demanding passage through Belgian territory, as German troops crossed the frontier of Luxembourg. Military operations began on the French frontier, Libau was bombarded by a German light cruiser SMS Augsburg and the British government guaranteed naval protection for French coasts. On 3 August the Belgian Government refused German demands and the British Government guaranteed military support to Belgium, should Germany invade. Germany declared war on France, the British government ordered general mobilisation and Italy declared neutrality. On 4 August the British government sent an ultimatum to Germany and declared war on Germany at midnight on 4/5 August, Central European Time. Belgium severed diplomatic relations with Germany and Germany declared war on Belgium. German troops crossed the Belgian frontier and attacked Liège.
Liège is situated at the confluence of the Meuse, which at the city flows through a deep ravine and the Ourthe, between the Ardennes to the south and Maastricht (in the Netherlands) and Flanders to the north and west. The city lies on the main rail lines from Germany to Brussels and Paris, which von Schlieffen and von Moltke planned to use in the invasion of France. Much industrial development had taken place in Liège and the vicinity, which presented an obstacle to an invading force. The main defences were in the Position fortifiée de Liège (Fortified Position of Liège), a ring of twelve forts 6–10 km (3.7–6.2 mi) from the city, built in 1892 by Henri Alexis Brialmont, the leading fortress engineer of the nineteenth century. The forts were sited about 4 km (2.5 mi) apart to be mutually supporting but had been designed for frontal, rather than all-round defence.