Belgium and the Franco - Prussian War Though not actually a belligerent in the Franco – Prussian War ( 1870 – 71 ) , Belgian society and politics were heavily affected by the conflict , and in particular the fear of invasion by either side . The mobilization of the army in 1870 highlighted the inadequacies of the Belgian military and led to reform of the system of conscription ( and the abandonment of the system of " Remplacement " ) and a programme of re - fortification towards the end of the 19th century which would greatly influence the early phases of the First World War . Initial panic and mobilisation . It was assumed by many that in the event of war between France and Prussia , an attack through Belgium by either side might reasonably be the first act , particularly after Emperor Napoleon III ' s attempt to annex Luxembourg in 1867 . Indeed , in the early part of the war , French Marshal Canrobert brought an entire Army Corps ( 4 infantry divisions ) to Châlons - sur - Marne in northern France as a reserve and to guard against any Prussian advance through Belgium . So when news of the declaration of war was received , the Belgian government of Jules d ' Anethan ( installed only two weeks before the war ' s outbreak ) , under King Leopold II , feared that it might be overrun . The gold reserves of the National Bank were hurried to the National Redoubt at Antwerp before the news became public . When this leaked out , it caused panic . The Belgian army was called up on the 15 July , the same day that both French and German armies mobilised . Many military leaders feared that , even after the outbreak of hostilities , as both French and Prussian armies manoeuvred on the Belgian border , one of them would seek a strategic advantage by an outflanking attack through Belgium and most believe the army incapable of fending off any such attack . Despite key battles taking place very close to Belgian territory , including the Battle of Sedan just a few miles from the border , Belgium was never actually attacked . A possible deciding factor in the hesitation of both sides to attack Belgium was the British guarantee of Belgian neutrality issued at the Treaty of London in 1839 . In order to avoid giving the impression of belligerence in the conflict , Leopold requested that the French not commit Belgian members of the French Foreign Legion during the conflict . The French agreed and Belgian legionnaires remained in their base in French Algeria while their comrades were deployed to the front . The decision outraged the other legionaries and the Legion ' s march , " Le Boudin " , makes repeated reference to the fact that the Belgians " [ only ] shoot from their rear - end " ( " " tireurs au cul " " ) because of it . Legacy . The Franco – Prussian War made Belgians acutely aware of the precarious situation of their country in the event of another war between the two powers . In the years following the conflict , there was widespread modernisation of the military . The system of " Remplacement " ( whereby wealthy Belgians conscripted into the military could pay for a " replacement " to do their military service instead ) which had been viewed as a crucial personal liberty by many Belgians was abolished and an improved system of conscription implemented . These reforms , led by d ' Anethan and under pressure from Leopold II , divided Belgian politics . The Catholics united with the Liberals under Frère - Orban to oppose them , and the reforms were finally defeated when d ' Anethan ' s government fell during an unrelated scandal . Eventually , the military was reformed . The 1909 System abolished the inefficient system of " Remplacement " , instituting compulsory military service of eight years ' service in the front lines and five years in the reserves . This swelled the size of the Belgian army to over 100 , 000 well - trained men . Construction of a chain of forts along Belgium ' s borders was intensified , and led to a series of very modern fortifications , including the so - called " National redoubt " at Antwerp , at Liège and Namur , many of them designed by the great Belgian fortress architect , Henri Alexis Brialmont .