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The history of the Mannerheim Line

From the early days of independent Finland to the Winter War

Part II


in Part I:

  • The start

  • The "Enckell Line"

  • The fortifying starts again

  • Building the line


Part II, includes:


in Part III:

  • The defenses in the "Summankylä"-sector

  • The concrete bunkers in Summankylä (table)

  • A panorama from the Summa village

  • Two photos taken of bunkers on the eastern side of the Summa village

  • The fields of fire from the concrete bunkers in the Summa village

  • The breaking of the line
        The Lähde sector
        The strongpoints in the Lähde sector, December 1939



The defensive lines in the Karelian Isthmus


This map shows the "Mannerheim Line" in dark blue, the "Intermediate Line" (Väliasema in Finnish) in slightly lighter blue, and the "Final Line" (or "Rear Line", Taka-asema in Finnish) in light blue.


The mobilization


When Finland called up the "Extra rehearsal" (YH-"Ylimääräiset harjoitukset" in Finnish), in early October, which was in fact the mobilization, the situation of the defensive fortifications in the Isthmus was far from good.

A shelter under construction (during the Mobilization)
Picture source: "Talvisodan Historia 1", p.230

A report was given by Lt.Col. V.K.Nihtilä about the situation in the Isthmus, and there was mentioned that the field fortifications (meaning trenches for infantry, wood & soil bunkers and shelters and wire obstacles) were NOT built, with the exception of some small works done in the previous summer, and that the AT-obstacles were "under construction" and should be finished before the year's end.

In the previous years, from late 20's onward, some short stretches of field fortifications (trenches and wire obstacles) had been made. This was done mostly by conscripts as a part of their training, but almost all of them were made as delaying positions near the lakes, south of Uusikirkko, and near Rautu. And to make matters worse, a very large part of the old wire defenses were almost completely decomposed.

All units deployed in the Isthmus began immediately hectic efforts to dig trenches, build wire obstacles and speed up the work of AT-obstacles.


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The Mannerheim Line ready and powerful?


During the peaceful years of 20's and 30's, the bunkers were often built as cheap as possible. For instance, only ONE machine gun bunker had the mg-pedestal installed (the factory prototype) and no weapons had been installed. So after the mobilization began, the first thing to do was to quickly design a new pedestal from wood and build them by hand. The bunkers were not camouflaged and only occasionally guarded, so the Soviet intelligence was able to find out the exact positions and layouts for most of the fortifications before the war. This was partly possible because no restrictions were made for people to visit the bunker sites. (It was common to find detailed sketches and maps about Finnish defenses from fallen Soviet officers.)

One has to remember that the Mannerheim Line did not exactly follow the "Enckell Line". So the bunkers of "Muolaa"(Mu), "Sikniemi"(Mu) and "Salmenkaita "(Mä) were actually not a direct part of Mannerheim Line. Instead as the line along the many isthmi between Lakes Muolaanjärvi and Vuoksi, that portion of the Mannerheim Line had only field fortifications.
So at the end of October 1939, when the war started, the actual Mannerheim Line had in the area between the Gulf of Finland and Vuoksi, some 80 km wide, 41 concrete fortifications (10 of them built in the 1920's and not repaired/modernized during the 1930's) and in the Vuoksi-Suvanto-Taipale area, some 55 km wide, 25 concrete fortifications (all of them built during the 1920's) see the map below. If the concrete fortifications had been dispersed evenly along the line, the distance between two of them would've been some 2 km, i.e. an average density of about 0.5 concrete bunkers / km.

And if counting concrete bunkers which had machine guns or cannons (thus excluding passive concrete shelters), a total of 48 bunkers, there was one such bunker for each 3 km of front, i.e. roughly 0,35 armed bunkers / km.

(Compare the above figure with the average number of concrete fortifications / km in the following lines;
- some 10 in the Maginot Line, in the weaker part at the Belgian border
- Westwall had at least 15)

As the primary AT-obstacle, from a wide variety of possibilities, the "rock obstacle"(also "rock rows") was chosen to be the primary type. There were several sound reasons for that. First, the soil of Finland has large deposits of granite (less expensive than concrete, which was on short supply). Second, the shortage of equipment capable of mixing the cement at the construction sites.

Picture source: "Talvisodan Historia 1", p.234


And third, the relatively low number of trucks in Finland (a nationwide total of 13 000), which forced to the use of carts drawn by horses, thereby limiting the practical transportation distance from quarry to construction site.
Some AT-ditches were made and some "slope cuts" ("rinneleikkaus" in Finnish) where in the base of the slope, soil was dug out to produce a vertical "wall".

The main problem to arise was that the obstacles were tested by Finnish Renault FT 17 and Vickers 6-ton tanks thus giving a wrong idea of their effectiveness (which was usually overestimated). There handicap became even greater when the ground was covered with snow.
The AT-obstacles of the Mannerheim Line were built in the most threatened areas, such as "Inkilä", Summa and the nearby areas, the isthmi between Lakes Muolaanjärvi and Vuoksi, and Taipale.

Also, the flooding of some areas was planned, but they produced only small results, partly because of the exceptionally cold winter.

The sheer volume of work and too small resources resulted in a line that practically lacked depth. Only in the Summa sector on the "Gateway of Karelia" was the back line (2 km from the front-line) of the main defense line with adequate field fortifications. In most places, tactical depth was achieved by locating some front-line unit shelters 200 -300 meters back from the front.

The Mannerheim Line was a line, which used natural barriers. The cold winter made this however in many cases a threat as the ice on the lakes and Ladoga and Gulf of Finland could carry the weight of Soviet light tanks (BT-series and T-26 types). This wouldn't have been a serious problem if

a) the Finnish artillery had had enough ammunition
b) there would've been more AT-guns, and
c) the testing of "ice mines" would've been conducted before 1939 (the bitter experience led to the development of the effective "Arsa-mines" after Winter War).

The result of the ice cover on the lakes and sea was, that Soviet tanks could easily launch attacks against Finnish isles and make flanking maneuvers, while the Finns tried to prevent this by both blowing and sawing gaps in the ice, which after a few hours froze up again.
Also, the frozen lakes provided the Red Army good supply routes and deleted the need of bridging equipment in many places (Taipale, for instance). In many places the Finns were forced to build temporary defensive positions on the ice of Lakes. These were built mainly of snow and logs, but soon pulp cubes were noted to be quite useful after first being watered and then allowed to freeze solid before covering them with snow. However on most cases the breastwork offered cover from small arms only, therefore the positions were manned during the dark hours only, the Soviet air superiority and the abundance of enemy artillery forced the Finns to withdraw from them before dawn.

Terrain features

The terrain was generally low characterized by small rises with height varying 15 - 40 meters.

The soil on the line consisted of loose soil types, covering the bedrock completely.

The soil in front of the line, if divided into marshy ground and firm ground, was
app. 30 % marsh
app. 70 % firm ground.

The land in front of the line, if divided into wood, cultivated and other, was
app. 75 % wood
app. 20 % cultivated fields
app. 5 % unused, open areas etc.

The trees in the woods, along the line, was relatively young.
app. 36 % of the trees were between 1 - 40 years of age
app. 53 % were 40 - 80 years
app. 10 % was over 80 years

If the woods, along the line, are divided by dominating tree type, then
app. 51 % was dominated by pine trees
app. 26 % by spruce trees
app. 20 % by deciduous trees
(general estimates)

In average, one hectare of forest had app. 63 cubic meters of timber (range 55 - 80 m3).

(Source: P.Hovilainen "Mannerheim-linja..." , p.6-7)

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It's indeed fair to say that the cornerstone of the Finnish defenses were the concrete fortifications, but unlike the Soviet propaganda during and after the war painted, the Mannerheim Line was light-years behind the massive fortress complexes of the Maginot Line. The strength of the Mannerheim Line was mostly based on common field fortifications, which were built during the summer of '39 and mobilization.

The maps on the right show "the greatness of the Finnish defenses on the Karelian Isthmus" according to Soviet sources. The defensive "lines" before the "Mannerheim Line" are close to the Finnish delay positions, but the few sparse fortifications in those positions don't deserve the name "defensive line".


The Mannerheim Line in figures

  • Total length approximately 135 km
    (if the start and end points of the line are connected by a straight line, the length is 96 km)

  • The amount of concrete used in the fortifications totaled 14 520 cubic meters of concrete.
    ( As a comparison, in the so-called "VT-Line", which was built by Finland during the 1941-1944 war and was positioned behind the front line in the Karelian Isthmus, some 400 000 cubic meters of concrete was used. That is over 26 times more concrete! And the VT-Line was considerably shorter.

    According to some 435 000 cubic yards of concrete has been used in the Pentagon. That equals some 332 500 cubic meters.

    Also, for example, some 15 500 cubic meters of concrete was used in the Opera House of the Finnish National Opera in Helsinki (, built in 1993. Thanks to Ville Virrankoski for forwarding me the information.

  • There were 11 roads which run through the Mannerheim Line



The concrete bunkers in the Mannerheim Line

(Map source: P.Hovilainen "Mannerheim-linja..." , attachments)

Notes:The small sticks represent machine guns. The small red dots represent cupolas.

Note also that the sections of the Mannerheim Line, which are not marked with those blue sector boxes, didn't have any kind concrete fortifications.


Machine gun bunkers
Cannon bunkers
Passive shelters
III Corps
II Corps

"number" = how many of them were there
"mg's" = how many machine guns these bunkers had in total
"obs.cup." = how many observation cupolas these bunkers had in total
"accomm." = how many men these bunkers could officially accommodate

(Source: P.Hovilainen "Mannerheim-linja..." , p.13)


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Field fortifications in the Mannerheim Line

Map of the sectors, detailed in the table below

passive shelters
trenches* / km
barbed wire / km
tank obst. / km

* = Figure include all trench types, communication trenches, front-line combat trenches etc.

(Source: P.Hovilainen "Mannerheim-linja..." , p.18)


The Isthmus Gateway, between river Summa and Lake Muolaanjärvi (a sector some 15 km wide), was the most heavily defended, and it had an average of 10 mg bunkers / nests, 7 shelters and 6 km of trenches (including the connection trenches) per kilometer of front.


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Go to Part I

Go to Part III


See also:

For questions about picture copyrights, see 'Sources' page

Copyright © 1999 - 2006 Sami H. E. Korhonen

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