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.
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. The Belgian troops were divided into two armies; the Army of Antwerp (15,000 men) was tasked with guarding the fortresses at Antwerp and across Belgium while the Army of Observation (55,000 men) was tasked with defending the national borders.
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.
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.
A commemorative medal, the 1870–71 Commemorative Medal, was inaugurated to the veterans of the conflict in 1911.