The relationship between the two countries deteriorated, with Soviet Russia supporting the Red Guard during the Finnish Civil War in 1918.

[2] The situation was considered dangerous for a new nation like Finland, especially as the capital of the new communist revolution was nearby Petrograd, (now Saint Petersburg).

During the civil war of 1918, the Finnish government and high command started to develop defence plans to protect against possible attacks from the Soviet Union.

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.

[5] The Germans had ordered Colonel Otto von Brandenstein to investigate defensive positions on the Karelian Isthmus; he delivered his plan on 16 July.

[6] During October 1919 Finnish Chief of Staff Major General Oscar Enckell sited the line, mostly following the original course that von Brandenstein had presented.

[7] Major J. Gros-Coissy, a member of the French military commission, designed the fortifications together with Finnish Lt Col Johan Fabritius.

Some bunkers' loopholes were simply closed-up as part of a plan to make them more suitable for accommodation or command posts.

The Finnish communist party, run from the Soviet Union, had its own military reporting line to the Central Committee.

The most important Soviet intelligence organisations in Finland were the NKVD and the Fourth Department of the Army General Staff.

Leningrad Military District, the Baltic Fleet and border troops under the NKVD conducted espionage operations.

Vilho Pentikäinen, a photographer serving on the Finnish general Staff, escaped to the Soviet Union in 1933.

Before the start of the Winter War, Soviet intelligence published a book for Red Army officers.

[1] The line ran from the coast of the Gulf of Finland in the west, through Summa to the Vuoksi River and ended at Taipale in the east.

[15] During the war, both Finnish and Soviet propaganda considerably exaggerated the extent of the line's fortifications[citation needed]: the former to improve national morale, the latter claimed it was stronger than the Maginot Line to explain the Red Army's slow progress against the Finnish defences.

[citation needed] Subsequently, the myth of the "heavily fortified" Mannerheim Line entered official Soviet war history and some western sources.

[citation needed] The vast majority of the Mannerheim Line simply comprised trenches and other field fortifications.

[16] The Finns originally aimed to make its defence line impregnable, however actual construction progress came nowhere close to this goal by the time the Winter War broke out[citation needed], in contrast to the Maginot Line which effectively deterred a cross-border assault.

The Finns had funds and resources for only 101 concrete bunkers; the equivalent length of the Maginot Line had 5,800 of these structures which were also linked by underground railway connections.

The main intention of flexible type field fortification was to close potential traffic and attack barriers with multiplied anti-tank ditches, hedgehogs, and dragon's teeth.

These were followed by a complex system of ditches and barbed wire obstacles, which protected the anti-tank barrier against sappers, bridge-layer tanks, and engineer teams.

Therefore, the enemy was forced to attack trenches as in World War I, at the cost of numerous losses, without armor and direct fire support.

[14] Maginot and Siegfried-like bunkers had numerous weaknesses, such as having destructible air inlets and firing holes, being too large (camouflage and costs), and yet being vulnerable to small sapper teams (at Sedan a few German soldiers destroyed several MG bunkers with pre-fabricated bombs and smoke grenades), and being blinded by small concentrated smoke screens.