A 200 km line of fortifications built to protect France's eastern borders with Germany (from the Belgian Ardennes to the Swiss border) and Italy in the 1920s and 1930s on the initiative of ministers André Maginot and Paul Painlevé; most people mentioning it are thinking exclusively of the section facing Germany.
In 1914, the French army, still sore about its defeat by the Prussians in 1870, had gone into the First World War committed to guerre à l'outrance, a policy of attack at any cost. Even the most fleeting acquaintance with the events of that war will be enough for the reader to realise that this was not exactly well-timed; France suffered massive losses in the "Battle of the Frontiers", its initial incursion onto German soil, before the Germans advanced through Belgium to the Marne and the static war on the Western Front began. At France's bloodiest battle, Verdun, the embarrassing loss of two key (but underdefended) fortifications, Fort Vaux and Fort Douaumont, was a major factor in prolonging the suffering. With two thirds of French war veterans having seen service in the carnage at Verdun (where Maginot himself had been wounded) and a looming manpower shortage for the field army due to the low birth rate during and after the war, the French were determined (or at least easily convinced) not to go into a subsequent war with their defences in such a weakened position.
The northern sector of the Maginot lines comprised 22 large and 36 smaller fortresses, mainly underground, plus 410 other fortifications, shelters and observation posts. The forts had interlocking fields of fire - each could fire directly at its neighbours to deal with attackers on the forts themselves - with a variety of turret- and casemate-mounted light artillery, mortars, anti-tank guns and machine guns, and were designed to withstand multiple hits from the sort of massive shells that had broken through the Belgian border forts in 1914. Although garrison troops had traditionally been chosen from weaker units, the Maginot line's defenders were carefully selected troops and technicians, the latter to run the very substantial power and logistics installations involved; these forts would not fall because of a failing water supply as Fort Vaux had in 1916.
Prior to the mid-1930s, Belgium - itself not short of border fortresses, notably at Liege and Eben Emael - was actively allied with France; the intention was to defend along their respective borders in the event of a German attack. It would not have been politically acceptable to fortify along the border with Belgium, since this would have indicated a de facto intention by the French to sacrifice their ally. Furthermore, since a large proportion of French industry was close to the French/Belgian border it made strategic sense to try and avoid losing it in the initial attack (as they had in 1914) by keeping the front line well into Belgium. Under the terms of the alliance, the French could have moved troops into Belgium before Belgium was invaded, in order to help hold the Germans in the east.
Unfortunately, following the French failure to resist the reoccupation of the Rhineland in 1936, the Belgians became nervous and withdrew from the alliance, declaring themselves neutral, meaning that the French would not be able to advance into Belgium until after the invasion had started. An extension of the line to the north-west of its original Northern end, Longuyon, was started, but by this time money was tight and only a few small fortifications were completed.
The result of this was that by the time the bulk of the French field army and the BEF moved into Belgium (incorrectly anticipating that the advance through Dutch Limburg and north of Liege was the main thrust of the attack, as it had been in the compromised Fall Gelb plan), they were unable to move fast enough to make use of the favourable defensive terrain in Eastern Belgium and were forced to try and set up a defensive line half way across Belgium, running roughly Breda-Antwerp-Leuven-Wavre-Namur, the last section of which (the "Gembloux gap") was completely open terrain apart from the cuttings and embankments of the railway line which passes about twenty metres in front of me as I write this*.
This defensive operation ("the Dyle manoeuvre") was successful enough at an operational level - the northern prong of the German assault was halted - but the delays and logistical problems caused by the late move into Belgium and the refugee crisis resulting form the overrunning of half the country exacerbated the failure to provide adequate cover for the Ardennes and Sambre-et-Meuse - the hilly areas now south of the bulk of the field armies and north of the Maginot line - allowing what was actually rather an against-the-odds operation by the Germans to come off substantially better than they had anticipated. Had the French already been in Eastern Belgium when the Germans attacked, the German advance would have met substantially stiffer resistance in the same area that the Battle of the Bulge was to be fought four and a half years later, and would have been unlikely to make it across the Meuse at Dinant and Sedan, as they did. At that point, with a Western Front returned to stasis, the Maginot Line would have been a fairly handy position.
The Line itself held out without difficulty (only one small fort fell to enemy attack) as the German Blitzkrieg through the Ardennes advanced into northern France and they were put under attack from the rear (despite numerous ignorant comments to the contrary, the forts' weapons covered front and rear, as had those of any serious fortification over the preceding couple of millennia except Singapore); although the garrison commanders intended to hold out to the end they were overruled by the government's decision to sign an armistice. The occupying Germans mostly used the Line for garrison purposes and stripped the equipment, although sections of the line around Metz were attacked by the Americans in 1944. The Line was once again manned after the war in anticipation of another attack from the East by the Warsaw Pact, but after the development of France's force de frappe in the 1960s, most of the installations were decommissioned. The majority of the forts remain in varying states of dereliction, although a number have been preserved and are open to visitors.
Contains information culled from the extensive site on
* Where I wrote the first version of this writeup. I've moved to the other end of the village since then; the house has some interesting angles as a result of damage from the shelling.