The Battle of the Ardennes took place during the First World War fought on the frontiers of France, Germany, Belgium and Luxembourg from 21 to 23 August 1914. The German armies defeated the French and forced their retreat. The battle was part of the larger Battle of the Frontiers, the first battle of the Western Front.

Belgian military planning was based on an assumption that other powers would expel an invader but the likelihood of a German invasion did not lead to France and Britain being seen as allies or for the Belgian government intending to do more than protect its independence. The Anglo-French Entente (1904) had led the Belgian government to think that the British attitude to Belgium was that it had come to be seen as a protectorate. A Belgian General Staff was formed in 1910 but the Chef d'État-Major Général de l'Armée, Lieutenant-Général Harry Jungbluth was retired on 30 June 1912 and only replaced in May 1914 by Lieutenant-General Chevalier Antonin de Selliers de Moranville, who began work on a contingency plan for the concentration of the army and also 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 river Gete with two divisions forward at Liège and Namur.

Field marshal Alfred Graf von Schlieffen was Chief of the German General Staff (Oberste Heeresleitung, OHL) from 1891 until his retirement in 1906. A student of Carl von Clausewitz, like other Prussian officers, he had been taught that "the heart of France lies between Paris and Brussels". In 1839, the Treaty of London masterminded by the British diplomat Lord Palmerston was signed by France, Prussia, Russia, Austria and the United Kingdom creating the independent Kingdom of Belgium. France and Russia joined in a military alliance in 1892, which threatened Germany with the possibility of a war on two fronts. German strategy gave priority to an offensive operation against France and a strategic defensive against Russia. Planning would be determined by numerical inferiority, speed of mobilisation, concentration and the effect of modern weaponry. The Germans expected frontal attacks to be costly and protracted, leading to limited success, particularly after the French and Russians modernised the fortifications on their frontiers with Germany. To evade the fortified frontier with France, Schlieffen devised a plan that by 1898–99 envisioned German forces rapidly passing between Antwerp and Namur to take Paris from the north, inflicting a quick and decisive defeat on France.

The German left flank in occupied Alsace would tempt the French into attacking there, drawing the French forces away from Paris and the German right. In its 1906 version, the Schlieffen Plan allocated six weeks and seven eighths of the Imperial German Army (a force of 1.5 million men) to overwhelm France while the remainder fought against the Russian Army in East Prussia. 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 to the plan, by adding divisions to the left flank opposite the French frontier, from the c. 1,700,000 men expected to be mobilised in the Westheer (western army). The main German force would still advance through Belgium and attack southwards into France, the French armies would be enveloped on the left and pressed back over the Meuse, Aisne, Somme, Oise, Marne and Seine, unable to withdraw into central France. The French would either be annihilated or the manoeuvre from the north would create conditions for victory in the centre or in Lorraine on the common border. Moltke planned for a force of about 320,000 men to defend Alsace-Lorraine south of Metz, 400,000 men to invade France and Luxembourg through the Ardennes and 700,000 more troops to invade Belgium.

After the defeat in the Franco-Prussian War, France had been humiliated, forced to pay the French indemnity of five billion francs and lost the provinces of Alsace and Lorraine to the new German Empire, so as to permanently put France on the defensive. The French fortified the common border but after 30 years the French felt confident enough to plan for an offensive strategy, thanks in no small part to Ferdinand Foch. France had a population and birth rate smaller than those of Germany and invented the concept of élan vital and decided on a strategy of "offensive to the limit", making the will to fight the cornerstone of French military planning. Colonel Louis Loyzeau de Grandmaison, took up Foch's doctrine and delivered two speeches before the École Militaire that set the foundations of Plan XVII, which was formally adopted in May 1913. French strategists took account of the possibility of envelopment by the German right and calculated that the more powerful the German right, the weaker the center and left would be. The French decided to concentrate their forces on the Rhine, planning to break the German left and center on either side of Metz, to cut off the German right and defeat the German armies in detail.

Under Plan XVII, the French peacetime army was to form five field armies of about two million men, with groups of Reserve divisions attached to each army and a group of reserve divisions on the flanks. The armies were to concentrate opposite the German frontier around Épinal, Nancy and Verdun–Mezières, with an army in reserve around Ste. Ménéhould and Commercy. Since 1871, railway building had given the French General staff sixteen lines to the German frontier against thirteen available to the German army and the French could afford to wait until German intentions were clear. The French deployment was intended to be ready for a German offensive in Lorraine or through Belgium. The French expected that the Germans would use reserve troops but also assumed that a large German army would be mobilised on the border with Russia, leaving the western army with sufficient troops only to advance through Belgium, south of the rivers Meuse and Sambre. French intelligence had obtained a 1905 map exercise by the German general staff, in which German troops had gone no further north than Namur and assumed that plans to besiege Belgian forts were a defensive measure against the Belgian army.

A German attack from south-eastern Belgium towards Mézières and a possible offensive from Lorraine towards Verdun, Nancy and St. Dié was anticipated; the plan was a development of Plan XVI and made more provision for the possibility of a German offensive through Belgium. The First, Second and Third armies were to concentrate between Épinal and Verdun opposite Alsace and Lorraine, the Fifth Army was to assemble from Montmédy to Sedan and Mézières and the Fourth Army was to be held back west of Verdun, ready to move east to attack the southern flank of a German invasion through Belgium or south against the northern flank of an attack through Lorraine. No formal provision was made for joint operations with the British Expeditionary Force (BEF) but discreet arrangements had been made between the French and British general staffs; during the Second Moroccan Crisis in 1911, the French had been told that six British divisions could be expected to operate around Maubeuge.