The 'Ligne Maginot' was the French major line of concrete fortifications, obstacles and weapon positions built by France on its side of the French borders with Switzerland, Germany and Luxembourg (1930/25 June 1940).

The 'Ligne Maginot' was not extended to the north as far as the southern side of the English Channel because the French authorities wished not to offend Belgium, which set great store on its policy of neutrality.

The 'Ligne Maginot' was perhaps the single greatest French response to the country’s terrible experience in World War I, and was constructed in the years leading to the start if World War II as the primary means of buying time for the French army the time it would need to complete its mobilisation in the event of attack, and also to make it possible for French ground forces to move into Belgium for a decisive confrontation with Germany. In its basic concept, therefore, the 'Ligne Maginot' reflected in the minds of the French the perceived success of static and essentially defensive combat in World War, and was extolled as the means of preventing any invasion from the east.

The 'Ligne Maginot' was believed to be impervious to most forms of attack, including bombing and the fire of artillery and tanks, and incorporated underground railways to allow the rapid movement of large numbers of troops and sizeable quantities of matériel under cover. The line also featured state-of-the-art living conditions for the men of its garrisons, and enjoyed the benefits of air conditioning and comfortable messes. Nevertheless, the line proved strategically ineffective during the Battle of France: instead of attacking the line’s defences frontally, the Germans invaded north-eastern France through the Low Countries, bypassing the line to the north. French and British officers had anticipated this: when Germany invaded the Netherlands and Belgium, they implemented preordained plans to create an aggressive front across Belgium and connected to the northern end of the 'Ligne Maginot'. The French line was weak in the area of the Ardennes hill region of forest and deep ravines, which the French considered to be terrain which the Germans would probably not attempt to cross. In 'Sichelschnitt', the Germans then took advantage of the weakness of this point to cleave the Franco-British defensive front, and the Allied forces to the north were forced into the 'Dynamo' evacuation from the Dunkirk area, leaving the forces to the south unable to mount an effective resistance to the German 'Rot' (iii) invasion of France.

The defences were first proposed by Maréchal de France Joseph Jacques Césaire Joffre, who had been commander-in-chief of the French forces from 1914 to 1916, but were opposed by modernists such as Paul Reynaud who favoured investment in armour and aircraft. Joffre had support from Maréchal de France Henri Philippe Pétain. The French government’s response to the argument was a series of reports and commissions, and it was André Maginot, the French war minister between 1922 and 1924, and then once more between 1929 and 1932, who finally convinced the government to invest in the scheme.

In January 1923, after Germany defaulted on the payment of reparations, the French [prime minister, Raymond Poincaré, ordered French troops to occupy the Ruhr region of Germany in response. In the resulting Ruhrkampf (Ruhr struggle) between the Germans and the French up to September 1923, the UK condemned the French occupation of the Ruhr, and a period of sustained Francophobia broke out in the UK, where Poincaré was attacked as a cruel bully punishing Germany with unreasonable reparation demands. The British, who openly championed the German position on reparations, applied intense economic pressure on France to revise its policies toward Germany and at a 1924 conference in London designed to settle the Franco-German crisis resulting from the Ruhrkampf, Ramsay MacDonald, the British prime minister, effectively pressured Edouard Herriot, the French prime minister, to make concessions.

The major conclusion drawn in France after the Ruhrkampf and the 1924 London conference was that France could not make unilateral military moves to uphold the terms of the 1919 Treat of Versailles. The French were also well aware of the contribution of the UK and its empire and dominions to the Allied victory of 1918, and believed that France would need British help to win another war, and therefore that they could not risk the alienation of the UK.

In 1927, the Allied Control Commission, responsible for German compliance with Part V of the Treaty of Versailles, was abolished as a goodwill gesture. In their final report the commissioners issued a strongly worded statement stating that Germany had never sought to abide by Part V and the Reichswehr had been engaging in covert rearmament all through the 1920s. Under the terms of the Treaty of Versailles, France was to occupy the Rhineland region of western Germany until 1935, but in fact the last French troops departed the Rhineland in June 1930 in exchange for German acceptance of the Young Plan, a US-sponsored programme for the settlement of post-World War I reparations. As long as the Rhineland was occupied by the French, the Rhineland served as a type of collateral under which the French would annex the Rhineland in the event of any German breaching of the treaty’s terms, such as rearmament in violation of Part V. The threat was powerful enough to deter successive German governments of the 1920s from any overt violation of Part V. French plans, as developed in 1919 by Maréchal de France Ferdinand Jean Marie Foch, the Allied commander-in-chief during 1918, were based on the assumption that in the event of renews war with Germany, the French forces in the Rhineland would undertake an offensive to seize the Ruhr. A variant of the Foch plan had been used by Poincaré in 1923 in ordering the French occupation of the Ruhr. French plans for an offensive in the 1920s were realistic, as the Treaty of Versailles had forbidden German conscription, and the Reichswehr was limited to 100,000 men.

Once the French forces left the Rhineland in 1930, such use of the Rhineland as collateral for German compliance was no longer available, so France from that time onward had perforce to depend on Germany’s word that it would continue to abide by the terms of the Treaties of Versailles and Locarno, which stated that the Rhineland would forever remain demilitarised. Given that Germany had engaged in covert rearmament with the co-operation of the USSR from a time as early as 1921, a fact which became public knowledge in 1926, and that every German government had gone out of its way to insist on the moral invalidity of the Treaty of Versailles on the grounds that it had been based upon the so-called Kriegsschuldlüge (war guilt lie) that Germany started World War I, the French did not believe that the Germans would allow the Rhineland’s demilitarised status to continue, and also that at some time in the future Germany would rearm in violation of the Treaty of Versailles, reintroduce conscription and remilitarise the Rhineland.

The decision of 1920 to embark on the construction of the 'Ligne Maginot' was thus a tacit French admission that without the French threat to the Rhineland, Germany would soon start to rearm, and that the terms of the Treaty of Versailles’s Part V possessed only the most limited of lifespans.

After the end of World War I, the economy of Germany was three times greater than that of France, Germany had a population of 70 million persons by comparison with France’s 40 million persons, and the French economy was hobbled by the need to reconstruct the enormous damage caused by World War I, in which German territory had seen little fighting. The French military leadership had little belief in France’s ability to win another war against Germany on its own, most especially in respect of an offensive war. French politicians knew that the victory of 1918 had been achieved because the British empire and the USA were allied with France and that, without the UK and USA, France would have been defeated. Now, with the USA isolationist and the UK steadfastly refusing any 'continental commitment' to defend France on the same scale as in World War I, the prospects of Anglo-American assistance in another war with Germany appeared at best to be problematical. The Treaty of Versailles did not call for military sanctions in the event of any Germany remilitarisation of the Rhineland or breaking of the terms of Part V. Moreover, while the Treaty of Locarno committed the UK and Italy to come to French aid in the event of a 'flagrant violation' of the Rhineland’s demilitarised status, it did not define 'flagrant violation', and the British and Italian governments refused in subsequent diplomatic talks to define such violation, a fact which led the French to place little hope in the receipt of British and Italian help should Germany remilitarise the Rhineland. Given the diplomatic situation in the late 1920s, the French foreign ministry informed the government that French military planning should be based on the worst-case scenario of France fighting the next war against Germany without British or UK help.

France had an alliance with Belgium and with the states of the 'cordon sanitaire', as the French system of alliances in eastern Europe was known. Although the alliances with Belgium, Poland, Czechoslovakia, Romania and Yugoslavia were appreciated in Paris, it was widely understood that this could in no way offset the absence of the UK and USA. The French armed forces were especially insistent that the population disparity between France and Germany made ant French offensive war of manoeuvre little more than suicidal as there would always be greater numbers of German divisions: the only viable alternative was a defensive strategy to counter German aggression. At the heart of this French military thinking was the assumption that Germany would not go to war without conscription, which would allow the German army to exploit Germany greater population. Without the natural defensive barrier provided by the Rhine river, the French military leadership argued that France needed a new defensive barrier to replace the river. The efficacy of prepared defensive positions had been conclusively demonstrated in World War I, when a few soldiers manning a single machine gun post could destroy large attacks, so the construction of a great defensive line with subterranean concrete shelters would pave the way for the optimum deployment of France’s more limited manpower resources.

Part of the French reasoning in support of the 'Ligne Maginot' was a result of France’s catastrophic losses in World War I, and their effect on the French population. The decline in the birth rate during and after the war, resulting in a national shortage of young men for conscription in the mid-1930s. Given the manpower shortage they faced, French planners had to rely more on older and therefore less capable reservists, who would require a longer time to mobilise and at the same time degrade the capabilities of French industry as they left their civilian occupations. Thus the construction of static defensive positions would not only to buy time but also economise on men by the defence of key areas with fewer and less mobile forces. In 1940, France deployed about twice as many men, totalling 36 divisions or about one-third of its strength, for the defence of the 'Ligne Maginot' in Alsace and Lorraine, whereas the Germans' opposing Heeresgruppe 'C' contained only 19 divisions, or slightly less than one-seventh of the forces committed in 'Gelb' and 'Sichelschnitt' for the invasion of the Low Countries and France. Reflecting the 'lessons' of World War I, the French military had developed the concept of la puissance du feu (firepower), emplaced artillery sheltered by concrete and steel, as the means it needed to inflict devastating losses on any attacker.

The French planning for war with Germany was based on the assumption that such a conflict would be a guerre de longue durée (war of long duration), in which the superior economic resources of the Allies would gradually degrade the German strength to manageable proportions. The fact that the Germans embraced the concept of the Blitzkrieg (lightning war), which provided the vision of a swift war in which Germany would win quickly via a knock-out blow, was a testament to the fundamental soundness of the concept of the guerre de longue durée, which was based on the fact that while Germany had the largest economy in Europe, it nonetheless lacked many of the raw materials necessary for a modern industrial economy, which rendered Germany vulnerable to a blockade, and also lacked the agricultural capacity to feed its population. The guerre de longue durée strategy called for the French to halt the expected German offensive aimed at securing a swift victory and thus paving the way to a war of attrition in which the Germans would become exhausted and the French would then be in the position to launch a strategic offensive to win the war.

Within this overall strategic concept, the 'Ligne Maginot' was designed to block the main German blow should it be delivered in eastern France, and to divert this main blow through Belgium, into which the French forces would advance to meet and stop the Germans. The Germans were expected to fight costly offensives, whose failures would quickly erode German strength, while the French waged a total war with the resources of France, its empire and allies mobilised for the war.

As well as the demographic reasons, a defensive strategy also served the requirements of France’s diplomacy toward the UK. The French imported 33% of their coal from the UK, and 32% of all imports through French ports were carried in British ships. Of French trade, 35% was with the British empire, and the majority of the tin, rubber, jute, wool and manganese required by France came from the British empire. About 55% of overseas imports arrived in France via the English Channel ports of Dunkirk, Calais, Boulogne, Dieppe, Le Havre, Cherbourg and St Malo. Germany had to import most of its iron, rubber, oil, bauxite, copper and nickel requirements, making maritime blockade a devastating weapon against the German economy. For economic reasons, the success of the guerre de longue durée strategy would require the UK to maintain a benevolent neutrality at the very least, but preferably to enter the war as an ally as British sea power could protect French imports while preventing German imports. A defensive strategy based on the 'Ligne Maginot' was thus a very good way to demonstrate to the UK that France was not an aggressive power and would to go to war only in the event of German aggression, a situation that would make it more likely that the UK would enter the war on the side of France.

Beginning in 1930, the 'Ligne Maginot' was built in several phases by the Service Technique du Génie (STG) overseen by Commission d’Organisation des Régions Fortifiées (CORF). The main part of the construction effort had been largely completed by 1939, and extended from Switzerland to Luxembourg with a considerably lighter extension extending to the Strait of Dover after 1934. The original construction did not cover the area chosen by the Germans for 'Sichelschnitt', namely the Ardennes, as a result of Belgian neutrality.

The 'Ligne Maginot' was designed to satisfy several purposes, including the checking of any surprise attack, the covering of the French army’s two/three-week mobilisation, the reduction of military manpower requirements, the protection of Alsace and Lorraine which had been returned by Germany to France in 1918 and included major industrial centres, the provision of a basis for an eventual French counter-offensive, the persuasion of the Germans to attempt its circumvention via Switzerland and/or Belgium, the pinning of the Germans while the main French strength was brought up to reinforce the line, the display of a non-aggressive French posture, the triggering of British support should Germany invade neutral Belgium, and the drawing of Belgium into the war by leaving it open to German attack.

The fortifications of the 'Ligne Maginot' were manned by specialised fortress infantry, artillery and engineer units. The infantry manned each of the fortresses' lighter weapons and constituted units which could operate outside the line’s defences should this be deemed necessary. Artillery troops operated the heavy guns, and the engineers were responsible for maintaining and operating other specialist equipment, including all of the line’s communications systems. These troops wore distinctive uniform insignia and considered themselves among the elite of the French army. During peacetime, the fortresses were only partially manned by full-time troops, and could be quickly supplemented by reservists living in the area, and intended for rapid mobilisation in an emergency.

Full-time troops allocated to the 'Ligne Maginot' were accommodated in barracks close to the fortresses. They were also accommodated in complexes of wooden housing adjacent to each of the fortresses, which were more comfortable than living inside the defences but not expected to survive wartime bombardment.

Training was effected at a fortress near the town of Bitche, built in a military training area and so able to host live-fire exercises of the type impossible elsewhere as the other parts of the line were located in civilian areas.

Although the name 'Ligne Maginot' suggests a comparatively thin linear fortification, the defences were in fact quite deep, varying from the Franco-German border to the rear area between 12.5 and 15.5 miles (20 and 25 km). The line comprised an intricate system of strongpoints, fortifications and military facilities such as border guard posts, communications centres, infantry shelters, barricades, artillery, machine gun and anti-tank gun emplacements, supply depots, infrastructure facilities and observation posts. These various structures reinforced a principal line of resistance made up of the most heavily armed ouvrages, which can be roughly translated as fortresses or major defensive works.

From east to west, that is from front to rear, the line comprised no fewer than 16 elements. First of these was the border post line consisting of blockhouses and strongpoints, which were often camouflaged as inoffensive residential homes, built within a few metres of the border and manned by troops whose task it was to raise the alarm in the event of a surprise attack and then to delay the German armour with pre-prepared explosives and barricades. Second was the outpost and support point line some 3.25 miles (5 km) behind the border and based on a line of anti-tank blockhouses intended to provide the first real line of resistance to armoured assault, sufficient to delay the Germans and allow the crews of the CORF ouvrages to ready themselves at their battle stations; these outposts also covered the main passages within the principal line. Third was the principal line of resistance beginning 6.25 miles (10 km) behind the border and itself shielded by anti-tank obstacles made of metal rails planted vertically in six rows, with heights varying from 2 ft 4 in to 4 ft 7 in (0.70 to 1.40 m) and buried to a depth of 6 ft 7 in (2.0 m), and extending right along the length of the line except in areas of dense forests, rivers or other nearly impassable terrain; behind the anti-tank obstacle system was an anti-personnel obstacle system made primarily of dense barbed wire; anti-tank road barriers also made it possible to block roads at necessary points of passage through the tank obstacles. Fourth were the infantry casemates, which were Jumelage de mitrailleuses bunkers each armed with two machine guns and 37- or 47-mm anti-tank guns. These could be single with a firing room in one direction or double with two firing rooms in opposite directions. These generally had two floors, with a firing level and a support/infrastructure level that provided the troops with rest facilities and services including power-generating units, reserves of water, fuel, food, ventilation equipment, etc.). The infantry casemates often had one or two cloches or turrets located on top of them and sometimes used for the emplacement of machine guns or observation periscopes. They were manned by 20 to 30 men. Fifth were the petits ouvrages, which were small fortresses used to reinforce the line of infantry bunkers. Each petit ouvrage generally comprised several infantry bunkers, connected by an underground tunnel network with attached underground facilities, such as barracks, electric generators, ventilation systems, mess halls, infirmaries and supply caches. Each petit ouvrage was manned by between 100 and 200 men. Sixth were the ouvrages, which were the fortresses that were the most important of the fortifications in the 'Ligne Maginot' and therefore based on the sturdiest construction and the heaviest artillery. Each ouvrage comprised at least six 'forward bunker systems' or 'combat blocks', as well as two entrances, and were connected by means of a network of underground tunnels that often featured narrow-gauge electric railways for transport between bunker systems. The blocks contained infrastructure such as power stations, independent ventilating systems, barracks and mess halls, kitchens, water storage and distribution systems, hoists, ammunition stores, workshops and stores of spare parts and food, and each ouvrage was manned by between 500 and more than 1,000 men. Seventh were the observation posts, which were each sited on hills to provide the best possible field of vision over the surrounding area. Their purpose was to locate the German advance and then to control the direct fire and/or correct the indirect fire of artillery, and also to report on the progress and location of important German units. These posts were large buried bunkers of reinforced concrete construction, and were fitted with armoured turrets containing high-precision optics. The posts were connected with the other fortifications by field telephone and radio equipment. Eighth was the telephone network, which connected every fortification in the 'Ligne Maginot' including bunkers, infantry and artillery fortresses, observation posts and shelters. Two telephone wires were placed parallel to the line of fortifications, providing redundancy in the event of a single wire being severed. There were places along the cable at which troops operating in the open could connect to the network. Ninth were the infantry reserve shelters. These were located some 545 to 915 yards (500 to 1000 m) to the rear of the principal line of resistance, and were buried concrete bunkers designed to house and shelter up to a company of infantry (200 to 250 men) and had such features as electric generators, ventilation systems, water supplies, kitchens and heating, which allowed their occupants to hold out in the event of an attack. They could also be used as a local headquarters and as a base for counterattacks. Tenth were the flood zones, which were natural hollows or rivers that could be flooded if it was deemed necessary to create additional obstacles to any German advance. Eleventh were the safety quarters built near the major fortifications so that the complement of an ouvrage could reach battle stations in the shortest possible time in the event of a surprise attack. Twelfth and thirteenth were the supply and ammunition dumps. Fourteenth was the narrow-gauge railway system, which was a network of 23.62-in (600-mm) gauge tracks to facilitate the reammunitioning and resupply of the ouvrages from supply depots up to 31.1 miles (50 km) distant. Fifteenth were the high-voltage transmission lines, which were initially above the ground but then buried, and connected to the civilian power grid to provide electrical power to the many fortifications and fortresses. And sixteenth was the heavy railway artillery, which locomotives brought to planned locations from which it could support the emplaced artillery located in the ouvrages and intentionally limited to a range of between 10,935 and 13,125 yards (10000 and 12000 m).

In all, the 'Ligne Maginot' comprised 142 ouvrages, 352 casemates, 78 shelters, 17 observation posts and some 5,000 blockhouses.

There were several types of armoured cloche, all of them of made of steel and in effect non-retractable turrets. The most common of these types was the Guetteur fusil-mitrailleur cloche (machine-gun sentry) position. This had three or four crenels or embrasures each for a rifles or machine gun, a direct-vision block, a binocular block or 1.98-in (50-mm) mortar. Some cloches carried a superimposed periscope. The 'Ligne Maginot' possessed 1,118 GFM cloches, and almost every block, casemate and shelter was surmounted by one or two GFM cloches. The JM cloches (jumelage de mitrailleuses, or twin machine guns) were the same as the GFM cloches except for the fact that they had only one opening, which was equipped with a pair of machine guns. There were 174 JM cloches in the 'Ligne Maginot' defences, which also incorporated 72 AM cloches (armes mixtes, or mixed weapons), each equipped with a pair of machine guns and one 25-mm anti-tank gun. Some of the GFM cloches were transformed into AM cloches during 1934. There were also 75 LG cloches (lance-grenade, or grenade launcher), which were almost wholly covered by concrete, with only a small opening for the launching of grenades in the local-defence role. Each of the 20 VP cloches (vision périscopique, or periscopic vision) could be fitted with one of several different type of periscopes. Like the LG cloches, they were almost completely covered by concrete. Finally, the VDP cloches (vision directe et périscopique, or direct and periscopic vision) were akin to the VP cloches except for the fact that they possessed two or three openings for the provision of a direct view. Consequently, they were not covered by concrete.

The 'Ligne Maginot' also included its equipment a large number of retractable turrets. These were of several types including 21 turrets with the 2.95-in (75-mm) modèle 1933 gun, 12 turrets with the 2.95-in (75-mm) modèle 1932 gun, one turret with the 2.95-in (75-mm) modèle 1905 gun, 17 turrets with the 5.31-in (135-mm) gun, 21 turrets with the 3.19-in (81-mm) mortar, 12 turrets for mixed weapons, seven turrets for mixed weapons and one 1.98-in (50-mm) mortar, and machine gun turrets.

Both static and mobile artillery units were assigned to the 'Ligne Maginot', the régiments d’artillerie de position comprising static artillery units, and the régiments d’artillerie mobile de forteresse (RAMF) comprising mobile artillery units.