The Meuse valley was a route by which France or Germany could be invaded and after the Franco-Prussian War, General Henri Alexis Brialmont fortified the valley at Liège and Namur, to deter France and Germany from violating Belgian sovereignty. The Fortified Position of Namur (FPN) was built from 1888–1892, about 7 kilometres (4.3 mi) from the centre of Namur, to a standard design of triangular and quadrilateral shapes, to minimize the number of defensive batteries in the fort ditches, with the point facing outwards.
On the left bank of the Meuse lay the modernised forts of Fort de Malonne, Fort de Saint-Héribert and Fort de Suarlée, the unmodernised Fort d'Emines and Fort de Cognelée and the modernized Fort de Marchovelette. On the right bank were Fort de Maizeret, Fort d'Andoy and Fort de Dave, all modernized. The obsolete Citadel of Namur in the town became redundant.
The forts were built of non-reinforced concrete but this could only be poured in daylight, which caused weak joints between each pour. A citadel was built and covered by 3–4 metres (9.8–13.1 ft) of concrete; caserne walls which were less vulnerable, had concrete of 1.5 metres (4.9 ft) thickness, inside a defended ditch 8 metres (26 ft) wide. The entrance had a long access ramp at the rear facing Namur, protected by a tambour with gun embrasures perpendicular to the entry, a rolling drawbridge retracting laterally over a 3.5 metres (11 ft) pit equipped with grenade launchers, an entrance grille and a 57-millimetre (2.2 in) gun firing along the axis of the gate.
The forts at Liège and Namur had 171 heavy guns, with each fort equipped with 5–8 Krupp guns of 120-millimetre (4.7 in), 150-millimetre (5.9 in) and 210-millimetre (8.3 in) calibre, which were the most modern armaments available in 1888, mounted in retractable armoured steel turrets made in France, Belgium and Germany. Three smaller retractable turrets were built in the triangular forts and four in the quadrilateral forts, with 57-millimetre (2.2 in) guns for short-range defence and 6–9 more 57-millimetre (2.2 in) guns were mounted in casemates to defend the ditches.
The Army of Alsace advanced cautiously, as part of the main French offensive the Battle of Lorraine, by the First and Second armies into the province of Lorraine. The French reached the area west of Mulhouse by 16 August and fought their way into the city by 19 August. The German survivors were pursued eastwards over the Rhine and the French took 3,000 prisoners. Joffre ordered the offensive to continue but by 23 August, preparations were halted as news of the French defeats in Lorraine and the Ardennes arrived. On 26 August, the French withdrew from Mulhouse to a more defensible line near Altkirch, to provide reinforcements for the French armies closer to Paris. The Army of Alsace was disbanded, the VII Corps was transferred to the Somme area in Picardy and the 8th Cavalry Division was attached to the First Army, to which two more divisions were sent later. The German 7th Army took part in the counter-offensive in Lorraine, with the German 6th Army and was then transferred to the Aisne in early September.
