The 'Mannerheim Line' ('Mannerheim-linja' in Finnish) was a Finnish defensive fortification on the Karelian isthmus built with the object of denying the USSR with a direct line of advance from the Leningrad area ro Viipuri and Helsinki (1939/44).

While this was never an officially designated name, during the 'Winter War' it became generally known as the 'Mannerheim-linja' after the Finnish army’s then commander-in-chief, Sotomarsalkka C. G. E. Mannerheim. Beginning soon after Finland had secured its independence, the line was constructed in two phases between 1920 and1924 and between 1932 and 1939. By November 1939, when the 'Winter War' began, the line was by no means complete.

After the October Revolution in the Russian Empire, the Finns had declared their independence in 1917. Although the Russian Soviet Republic recognised Finland’s independence, the Finns did not trust the sincerity of this Soviet acquiescence. The relationship between the two countries deteriorated, with Soviet Russia supporting the Red Guard during the Finnish Civil War in 1918. After the victory of the White Guard, a group of Finnish communists fled to Soviet Russia and established the Communist Party of Finland.

The situation was considered dangerous for a new nation like Finland, especially as the capital of the new communist revolution was nearby Petrograd (ex-St Petersburg and now once again St Petersburg). Furthermore, before the Treaty of Tartu in 1920, the border area was restless. A senior officer of the former Imperial Russian army, General Leytenant Mannerheim, strongly opposed the Bolsheviks (Communists). Construction work on the Karelian isthmus had already begun when the Bolsheviks won the Russian Civil War in 1922.

During the civil war of 1918, the Finnish government and high command started to develop defence plans to protect their fledgling nation against possible attacks from the Soviet Union. The main such route was clearly the Karelian isthmus. The most endangered part of that isthmus was to the south-west, near the Gulf of Finland, whereas the north-eastern part was better protected by the natural waterways of Vuoksi, Suvanto and Taipaleenjoki. The first plans for a defensive line were commissioned by Mannerheim from a Swedish volunteer, Överstelöjtnant A. Rappe, at the beginning of May 1918. Rappe’s line was placed close to the border and designed to protect two railway lines that crossed the border and could be used in a counterattack toward Petrograd. When Mannerheim resigned at the end of May, Rappe’s plans were abandoned.

The young nation possessed no guard troops and the border area was insecure. Security of the border on the Karelian isthmus in June 1918 was the responsibility of the 2^nd Division and local White Guard units, which were also entrusted with the security of the fortification’s construction, which took the initial form of weak depots without any concrete.

The Germans had ordered Oberst O. von Brandenstein to investigate defensive positions on the Karelian isthmus, and von Brandenstein delivered his plan on 16 July. He was the first to suggest using, as defensive positions, the lake isthmuses, where smaller lakes such as the Kuolemajärvi, Muolaa and Suvanto lakes, as well as the Taipaleenjoki river, divided the Karelian isthmus to the shorter land sections, and his plan was initially approved by the Finnish high command in August 1918. In October 1918 the Finnish government allocated 300,000 marks for the work, which was to be carried out by German and Finnish sappers as well as Russian prisoners of war. However, the money allocated was insufficient and the lack of materials and a qualified workforce hampered the building of proper fortifications. With Germany’s defeat in World War I, von Brandenstein’s plan was scrapped.

During October 1919 Finnish chief-of-staff, Kenraalimajuri Oscar Enckell sited the line mostly along the original course that von Brandenstein had presented. Commandant J. Gros-Coissy, a member of the French military commission, designed the fortifications together with Finnish Everstiluutnantti Johan Fabritius. During the first period of construction, Fabritius suggested moving the defensive line farther to the south-east, and while the general staff discussed the issue, Enckell’s earlier plans were followed. Furthermore, insufficient funds resulted in a disagreement between the officers and Enckell resigned in 1924, and construction work was interrupted for a long period.

The prime contractor of the fortifications was the Finnish construction company Ab Granit Oy. The first 100 small bunkers were built between 1920 and 1924. For reasons of cost, the first-phase hard fortifications were of unreinforced concrete, which provided only limited protection as the compression density of the concrete was too low to offer resistance to anything larger than medium artillery.

The second phase of construction began on 1 April 1934, with Fabritius in command of the work. He designed two new kinds of bunker, Ink 1 and Ink 2. The bunkers were intended primarily for troop accommodation, but loopholes were crafted into armour plate in 1938 and 1939. A typical bunker was 49.2 to 65.6 ft (15 to 20 m) in length and 16.4 to 19.7 ft (5 to 6 m wide. A pioneer battalion constructed six bunkers in the Inkilä sector.

Between 1932 and 1938 the defence budget was so limited that the Finns could construct only two or three bunkers per year. In 1936 and 1937 they constructed two large strongpoints, Sk 10 and Sj 4 in the Summankylä and Summajärvi areas. Two smaller bunkers, Le 6 and Le 7, in the Leipäsuo sector, and Ink 6, in the Inkilä sector, were also built. The new bunkers differed from earlier designs in that their troop accommodation was located between the gun chambers, thus saving the cost of expensive reinforced concrete; the roof was protected by 6.6 to 9.8 ft (2 to 3 m) of earth and 3.3 to 9.8 ft (1 to 3 m) of stone rubble.

The Karelian fortifications received greater funding and more resources from May 1938 as the European political situation worsened. During this period the Finns built new strongholds and modernised old strongpoints. In Summakylä and Summajärvi they built two large Sk 11 bunkers, a 'Peltola', a Sj 5, a 'Miljoonalinnake', and an incomplete third Sk 17. These bunkers had better fire positions and ventilation, as well as a observation cupola.

In Suurniemi, near Muolaanjärvi, the Finns started the construction of seven new bunkers, Su 1 to Su 7. Two others, Su 3 and Su 4, were for accommodation, and the rest were machine gun nests. The Finns also modernised those structures built in the 1920s. The older bunkers were given added flank-fire capability and enlarged. Some bunkers' loopholes were simply closed-up as part of a plan to make them more suitable for accommodation or command posts.

The line was still incomplete in November 1939.[1]

Throughout this period, Soviet intelligence worked in Finland at several levels. The Finnish communist party, run from the USSR, had its own military reporting line to the Central Committee. Its intelligence concentrated on the Finnish army, recording the locations of Finnish artillery and defensive positions. The most important Soviet intelligence organisations in Finland were the NKVD and the Fourth Department of the Army General Staff. The Leningrad Military District, the Baltic Fleet and border troops under the NKVD also conducted espionage operations.

The Finns exposed two espionage cases during the 1930s: Vilho Pentikäinen, a photographer serving on the Finnish general staff, escaped to the USSR in 1933, and the second case was of Simo Haukka, who photographed and measured roads and terrain for Soviet intelligence in 1935.

Soviet intelligence published a top-secret and very detailed photobook of the Finnish terrain and fortifications in 1938. The book included a seven-page report and 22 pages of maps and photographs. Every issue was numbered, running probably only into dozens. Soviet intelligence activity increased in 1938 and expanded still further in 1939. Before the start of the 'Winter War', Soviet intelligence published a book for Soviet army officers. It was called Finland. Written Description of March Routes, and this was later translated and republished as the Red Army March Guide to Finland, which included more than 200 pages of maps and photographs.

Along with the intelligence, the USSR received a detailed map of the defences on the isthmus, which a German military attaché in Helsinki, General Arniké, delivered to the Soviets in September 1939.

The 'Mannerheim-linja' extended from the coast of the Gulf of Finland in the south-west, through Summa to the Vuoksi tiver and ended at Taipale in the north-east on the shore of Lake Ladoga. It comprised 157 machine gun positions and eight artillery positions built of concrete. The area around Summa was the most heavily fortified because it was thought to be the most vulnerable position.

The coast of the Gulf of Finland was guarded by Fort Saarenpää and that of Lake Ladoga by Fort Järisevä. These coastal artillery positions had 5-, 6- and 10-in (127-, 152- and 254-mm) guns.

Unlike the French 'Ligne Maginot' and other similar forts made with huge bunkers and lines of dragon’s teeth, the 'Mannerheim-linja' was built mostly by utilisation of the natural terrain. Many items such as fallen trees and boulders were incorporated into defensive positions. The Finns also mastered camouflage techniques, which they put to good use.

The 'Mannerheim-linja' was not constructed at huge expense, for its primary purpose was to delay rather than repel a Soviet invasion. The line was thus based on the methodology of the flexible defence, so it employed trenches and obstacles rather than large bunkers.

In the 'Winter War', the 'Mannerheim-linja' halted the Soviet advance for two months. The Soviet battleships Marat and Oktyabrskaya Revolutsiya delivered gunfire attacks on Fort Saarenpää several times during December 1939 and January 1940, but the Finns repelled the attacks, driving off Oktyabrskaya Revolutsiya with near misses on 18 December 1939.

During the war, both Finnish and Soviet propaganda considerably exaggerated the extent of the line’s fortifications: the former to improve national morale, and the latter to claim that was stronger than the 'Ligne Maginot' and thereby explain the Soviet army’s slow progress against the Finnish defences. Subsequently, the myth of the 'heavily fortified' 'Mannerheim-linja' entered official Soviet war history and some western sources. The great majority of the 'Mannerheim-linja' comprised simply trenches and other field fortifications. Bunkers along the line were mostly small and widely divided, and the line possessed almost no artillery.

After the 'Winter War', Soviet combat engineers destroyed the remaining installations. In the 'Continuation War' the line was not refortified, although both the Soviets and the Finns used its natural benefits in defence during the Finnish advance in 1941 and the Soviet offensive in 1944.

The first month of the Finnish campaign was a Soviet humiliation. By the third week of the war, Soviet propaganda was working hard to explain the Soviet army’s failure to the Soviet populace, and claimed that the 'Mannerheim-linja' was stronger than the 'Ligne Maginot'. The Finns had originally aimed to make its defence line impregnable, but construction progress came nowhere close to this goal by the time the 'Winter War' broke out, by contrast with the 'Ligne Maginot', which effectively deterred a cross-border assault. The Finns had funds and resources for only 101 concrete bunkers, whereas the equivalent length of the 'Ligne Maginot' had 5,800 of these structures, which were also linked by underground railway connections.

However, 'flexible' defence lines such as the 'Mannerheim-linja' were not based on dense lines of concrete bunkers and pillboxes, for the main object of the flexible type field fortification was to close potential traffic and attack barriers with complexes of anti-tank ditches, hedgehogs and dragon’s teeth. These were followed by another complex system of ditches and barbed wire obstacles, which protected the anti-tank barrier against sappers, bridgelaying tanks and teams of combat engineer. Thus flexible defence was intended to force an opponent to attack trenches, as in World War I, at the cost of numerous losses, without armour and direct fire support. It was termed flexible defence because the defending troops were not 'locked' into bunkers, but could be regrouped between field fortifications such as wood and earth firing posts, dug-outs and pillboxes, and would also have the option of carrying out a counterattack. All soldiers and weapons had multiple firing positions in order to make it difficult to keep them under fire. The concrete bunkers were usually only shelters, and were side-firing in order to defend anti-tank obstacles.

'Ligne Maginot' and 'Siegfried-Linie' bunkers had numerous weaknesses, such as having destructible air inlets and firing ports, being too large to camouflage and yet being vulnerable to small sapper teams (at Sedan a few German soldiers destroyed several French machine gun bunkers with prefabricated bombs and smoke grenades), and being blinded by small concentrated smoke screens. Flexible defence lines were almost immune to small sapper teams or small smoke screens, and had no easily targetable elements.