The 'Mannerheim-linja' was a Finnish defensive fortification line on the Karelian isthmus designed and constructed to aid Finland in defeating any Soviet aggression (1920/1940).
This defensive line was constructed in two phases between 1920 and 1924, and between 1932 and 1939. By November 1939, however, when the 'Talvisota' winter war began, the line was by no means complete.
After the revolution of October 1917 had brought the Bolsheviks and their allies to power in Russia, the Finns declared their independence, and despite the fact that Soviet Russia recognised Finland’s independence, the Finns did not trust the sincerity of the Soviets. The relationship between the two countries deteriorated, and Soviet Russia supported the Red Guard communist faction 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 dangerous for a new nation like Finland, especially as the capital of the new communist state was initially located in nearby Petrograd (Leningrad from January 1924 and now St Petersburg). Furthermore, before the Treaty of Tartu in 1920, the border area was restless. A former lieutenant general in imperial Russian service, Carl G. E. Mannerheim, became commander of the Finnish forces and was strongly opposed to Bolshevism. During the Russian Civil War, in 1919, the Finns even considered supporting an assault of the White Guards on Petrograd, but turned against the notion as the military situation in the Russian Civil War changed, and as they also believed that the Whites would not uphold Finland’s independence.
Construction work on the Karelian Isthmus had already begun when the Bolsheviks won the Russian Civil War in 1922. It was in the course of the civil war in 1918 that the Finnish government and high command had begun to develop defence plans to protect possible attack routes to Finland from Soviet Russia. The main such route was clearly the Karelian isthmus. The most endangered part of that isthmus was to the west, near the Gulf of Finland, for the eastern part was better protected by the natural waterways of the Vuoksi, Suvanto and Taipaleenjoki river and associated lakes.
The first plans for a defensive line were commissioned by Mannerheim from the Swedish volunteer Lieutenant Colonel A. Rappe at the beginning of May 1918. Rappe’s line was placed close to the border and designed to protect two railway lines which crossed the border and could be used in an attack 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 wholly insecure. Border security on the Karelian isthmus was the responsibility of the 2nd Division and local White Guard units in June 1918, and these were also entrusted with the security of the construction of the fortifications. The first such undertaking took the form of weak depots with earth rather concrete defences.
The Germans had agreed to allow Oberst O. von Brandenstein to investigate defensive positions on the Karelian Isthmus, and when he delivered his plan on 16 July, von Brandenstein became the first officer to suggest a defensive scheme based on the use of lake isthmuses, between smaller lakes such as Kuolemajärvi, Muolaa and Suvanto, as well as the Taipaleenjoki river. These split the Karelian isthmus into a number of shorter land sections. von Brandenstein’s 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 financial resources allocated to the task were insufficient, and a lack of building materials and a qualified labour force hampered the building of proper fortifications.
With Germany’s defeat of November 1818 in World War I, von Brandenstein’s plan was scrapped.
During October 1919 the Finnish chief-of-staff, Kenraalimajuri Oscar Enckell, became responsible for the siting of the line, and in general adhered to the original course proposed by von Brandenstein. Commandant J. Gros-Coissy, a member of the French military commission in Finland, and the Finnish Everstiluutnantti Johan Fabritius, designed the fortifications. During an early stage of the construction, Fabritius suggested the relocation of the defensive line farther to the south-east. The general staff discussed the matter, but then opted to adhere to Enckell’s earlier plan. Furthermore, the shortage of funding led to a disagreement between the officers, and Enckell resigned in 1924, triggering a long interruption of the construction work.
The prime contractor for the construction of the fortifications was the Finnish construction company Ab Granit Oy. The first 100 small bunkers were built between 1920 and 1924, and for cost reasons the hard fortifications of this first phase of construction were of unreinforced concrete, which provided only limited protection as the concrete’s compression density was too low to offer resistance to anything larger than medium artillery.
The seconds phase of the construction work began on 1 April 1934, with Fabritius as supervisor. This officer designed two new kinds of bunker, Ink 1 and Ink 2. The bunkers were designed primarily for troop accommodation, but loopholes were insinuated into armour plate in 1938 and 1939. Each bunker was between 49 ft 3 in (15.0 m) and 65 ft 7 in (20.0 m) long, and between 16 ft 5 in (5.0 m) and 19 ft 8 in (6.0 m) wide. A pioneer battalion constructed six such bunkers in the Inkilä sector.
Between 1932 and 1938 the defence budget was so straitened that the Finns could undertake the construction of only two or three such bunkers per year. In 1936 and 1937 the Finns also built 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 inasmuch as their troop accommodation was located between the gun casemates, thereby limiting the need for expensive reinforced concrete. The roof was protected by 6 ft 7 in (2.0 m) to 9 ft 10 in (3.0 m) of earth and 3 ft 3 in (1.0 m) to 9 ft 10 in (3.0 m) of stone rubble.
The work on the Karelian defensive fortifications received considerably greater funds and resources from May 1938 as the European political situation worsened. The Finns built new strongpoints and modernised the old positions. In Summakylä and Summajärvi the Finns built two large Sk 11 bunkers, an Sj 5 and an incomplete third Sk 17. These bunkers had better protection and ventilation, as well as an observation cloche.
In Suurniemi near Muolaanjärvi, the Finns started the construction of seven new bunkers as Su 1 to Su 7. Su 3 and Su 4 were intended for accommodation, and the others were machine gun positions. They also modernised the structures built in the 1920s. The older bunkers were upgraded with flank-firing capability, and were also enlarged. The firing ports of some bunkers were closed as part of a plan to make them more suitable for accommodation or service as command posts.
The 'Mannerheim-linja' was still incomplete in November 1939.
During the period between the two world wars, there were several levels of Soviet intelligence at work in Finland. 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, taking notes on the locations of Finnish artillery and defensive positions. However, the most important Soviet intelligence organisations in Finland were the NKVD (People’s Commissariat for Internal Affairs) and the Fourth Department of the Army General Staff. The Leningrad Military District, the Baltic Fleet and border troops under command of the NKVD all undertook espionage operations.
The Finns exposed two major cases of spying during the 1930s. Vilho Pentikäinen, a photographer serving on the Finnish general staff, then managed to escape to the USSR in 1933, and Simo Haukka took photographs and measured roads and terrain for Soviet intelligence in 1935.
Soviet intelligence published a very detailed photograph-illustrated handbook of the Finnish terrain and fortifications in 1938. Intended for senior officers and produced in only small numbers, the book included a seven-page report and 22 pages of maps and photographs. Soviet intelligence activity was increased in 1938 and was then expanded still further in 1939. Before the start of the 'Talvisota' winter war, Soviet intelligence published a book for the use of Soviet officers: this Finland – Written Description of March Routes was later republished as the Red Army March Guide to Finland, and included more than 200 pages of maps and photographs.
The USSR also received a detailed map of the defences on the isthmus from a German military attaché in Helsinki, who handed this to the Soviets in Moscow during September 1939.
The 'Mannerheim-linja' extended from the coast of the Gulf of Finland in the west, through Summa to the Vuoksi river, and ended at Taipale in the east. In overall terms it comprised 157 machine gun positions and eight artillery positions built of concrete. Facing what was believed to be the greatest Soviet threat, the area around Summa was the most heavily fortified part of the line.
In the south-west the Gulf of Finland coast was guarded by Fort Saarenpää, and in the north-east the western shore of Lake Ladoga by Fort Järisevä. These coastal artillery positions had artillery in calibres of 5, 6 and 10 in (127, 152 and 254 mm).
Unlike the French 'Ligne Maginot' and other similar fortifications, characterised by huge bunkers and lines of dragon’s teeth anti-tank defences, the 'Mannerheim-linja' was built largely by exploitation of the natural terrain. Much use was made of items such as fallen trees and boulders, which were incorporated into the defensive positions, and the Finns also proved themselves to be masterful exponents of an assortment camouflage techniques to help their defences blend into the terrain.
The 'Mannerheim-linja' was not constructed at great expense, and its purpose was to delay more than defeat any Soviet invasion.
In the 'Talvisota' winter war, the 'Mannerheim-linja' defences checked the Soviet advance for two months. Fort Saarenpää was attacked by the Soviet battleships Marat and Oktyabrskaya Revolutsiya on several occasions during December 1939 and January 1940, but the Finns repelled the attacks, driving off Oktyabrskaya Revolutsiya by near misses on 18 December.
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 explain their troops' slow progress against the Finnish defences. Subsequently, the myth of the 'heavily fortified' 'Mannerheim-linja' entered Soviet official war history, and thence some western accounts. The truth remains that the vast majority of the 'Mannerheim-linja' comprised only trenches and field fortifications. The line’s bunkers were mostly small and were also spread thinly. Moreover, the line incorporated very little in the way of artillery.
After the end of the 'Talvisota' winter war, Soviet combat engineers destroyed the remaining installations. In the 'Jatkosota' continuation war the line was not refortified, although both the Soviets and Finns used its natural features, the former during the Finnish advance of 1941 and the latter during the Soviet offensive in 1944, in which the newer 'VT-linja' and 'VKT-linja' also played a major role.