History of the Mannerheim Line -From early days to Winter War

Note! The Finnish Main Defense Line was baptized "Mannerheim Line" by foreign journalists. The name was adopted quickly by the troops and later on officially. So the Line was not baptized by a narcissistic Commander-in-Chief.

The defensive lines in the Karelian Isthmus
The mobilization
Mannerheim Line ready and powerful?
The breaking of the line

The history of the Mannerheim Line date back to the days of the Finnish civil war (also known as the war of independence) when the Finnish CinC of the White Army, General C.G.Mannerheim, began to make plans about the defense of the Karelian Isthmus (also in the text as just "Isthmus") against Russia.

On May 7th 1918, Mannerheim gave an order to investigate and make preliminary plans for defensive positions. A preliminary plan was made by two swedes, Lt.Col. A.Rappe and Major K. von Heijne. It was finished and delivered to Mannerheim's HQ on June 1st 1918. The plan was eventually ignored, as Mannerheim resigned from the position of CinC on May 29th, shortly after the plan was ready. An era of German influence started in the High Command of the Finnish Army.

This map shows the Main Defense lines in both Lt.Col. Rappe's (darker blue near the border) and Col. von Brandenstein's plans (light blue). Note that both plans incorporated also forward positions and rear lines, but in order to keep the map more readable I chose not to include them.

The next proposal about the position of the soon to be built defense line was made by a German Colonel, Baron O. Von Brandenstein. It's interesting, that the German plan was a lot more defensive in nature than the plan made by the Swedes. In fact, the line proposed, was very close to the eventually build Mannerheim Line. This line was also discarded, but it introduced the idea of using the Vuoksi - Suvanto lakes as a natural barrier, making also better use of the many lakes dotting the isthmus (the era of German influence ended when the German officers left the country at the end of the year).

On September 16th 1919, Major General P.O.Enckell became the Chief of general staff in the age of 41. He began immediately to study the different proposals and made his first personal trip to different sites on his planned defense line in late September.
There was a lot of discussion between high ranking officers of the Finnish army about the very idea of making passive fortifications thereby giving the psychological advantage to the enemy (note that a very big portion of the Finnish officers were very offensive-minded, like their German teachers).
But the cruel fact was, that Finland was absolutely too poor, and lacked the manpower to defend the Karelian Isthmus without fortifications. Besides, the use of fortifications decreased the number of men needed for defense, thereby freeing more men to conduct active operations against the enemy.

The idea of the "Enckell Line" was to create a chain of strongpoints, that by the use of carefully placed automatic weapons (firing from flank fire positions), the enemy was to be stopped by a relatively small number of men. Their most important objective was to buy time for the mobilization of the army (at that time the Finnish "army" could muster 3 � divisions without trained reserves) and to protect the transportation of the army to defend Viipuri.

There was one person, who contributed much to the overall fortifying of Finland before Winter War. This person was Lt.Colonel J.Chr.Fabritius, who had graduated from the military academy of Hamina and the Military-Engineer School in St.Petersburg, and had served as a "fortifications officer" (I'm not sure about that translation) in the Russian Army, and was also a civilian engineer. He made some proposals about the defense line.
The biggest difference between those two plans was the position of the frontline in the middle of the Isthmus (the area between lake Kuolema - lake Muolaa - Vuoksi). But Fabritius told his proposals to General Thesl�f, and according to Lt. General Enckell, the proposals didn't reach Enckell before the order to start the fortification works was given.

The Enckell Line and the proposal of Fabritius

The "Enckell Line" is in dark blue, and the proposal by Fabritius in lighter blue. The major difference in Fabritius's plan was that a defensive position 8 km deep would've been created to protect the "Karelian Gateway"(the shaded area) and that the isthmi positions between Vuoksi and lake Muolaa would provide a good base for active operations in either the Rautu-direction or towards Kuolemajarvi and Uusikirkko. Again, the map shows only the main defense lines (but since the Enckell Line's fortifications near Viipuri were built, I have included them also).

In fall 1919 a French military commission arrived to help in the creation and organization of the Finnish army. The commission was led by Col. G.Gendre and the fortifications expert was Major J.J.Gros-Coissy. Gros-Coissy was given the task of making more detailed plans of the fortifications on the Isthmus, and Fabritius (who was at the moment an employee of Oy Granit Ab) was asked to his help by the Minister of War. The co-operation between these two men began on October 21st 1919.

The first phase was to secure the most threatened spots between the Gulf of Finland and Vuoksi by mg-bunkers (in flank fire positions and interlocking fields of fire). The cost of one bunker was estimated to be 100 000 - 110 000 Marks and the needed number was 80 bunkers. But right from the start, the Ministry of Defense could only give 6 000 000 Marks for the first phase (a maximum of 60 bunkers). Gros-Coissy solved the problem by proposing that the bunkers would be built as front-firing bunkers (with a field of fire between 90 - 120 degrees). Fabritius strongly protested, but Enckell favored the Gros-Coissy's solution.
Enckell insisted also, that the bunkers should have overhead protection from artillery and that it should withstand three direct hits by 6 inch howitzers. This made the overall height of the bunker's front side 2,50 - 2,60 metres, making it difficult to camouflage them. The used concrete had an average compression strength of 300 kg/cm� (in 1939, the required compression strength was 450 kg/cm�). The walls of the bunkers were mostly made of "spare concrete" (the concrete had no steel reinforcement, instead rocks were mixed with concrete).

A total of 164 concrete fortifications (111 mg-bunkers with 1 mg, 3 mg-bunkers with 2 mg, 1 gun & mg-bunker, 6 gun-bunkers, 6 fire control bunkers, 27 passive shelters, 10 "concrete trenches") were made during 1920 - 1924. While Fabritius can be called as the second "creator" of the Enckell line, he also criticized it quite a lot. He wanted more depth and he also made remarks about positioning lone bunkers in the middle of fields with no cover. He was not alone with his criticism. Also the future General �hquist criticized the front-firing bunkers, calling them deathtraps.

On September 18th 1924, General Enckell resigned from his office. There were many reasons for his resignation, but one of them was the criticism, that the idea of fortifying the Isthmus encountered (the critics accused the fortifications of "killing" the active/offensive tactics of the army). The built line of fortifications was called the "Enckell Line" and after 1924 no new fortifications were build in the Isthmus for a long time.

In summer 1927, Captain V.A.M.Karikoski, made inspection tour to the small isthmi in the area between Vuoksi and lake Muolaa. After he left his report, dated August 12th 1927, it was shortly followed by a report from Major E.Voss on August 30th. Both proposals didn't lead to any actions, but it shows that the general staff was interested again.

After many proposals and inspection tours, in late 1931, the future Main Defense Line was decided to be built. It was decided that it would follow the plans of the "Enckell Line", with the exception of using plans by Fabritius in the area between lake Muolaa and Vuoksi.

The construction started again, after a 8 year pause, in mid-1932. The construction was concentrated in the Inkil� (Ink) area, and was performed by the Pioner (engineer) Battalion as a part of training, in order to save costs.

Fabritius was again involved in his beloved task of building fortifications in the Isthmus to defend the country. He was appointed as the chief of the new "fortifications office" (Linnoitustoimisto) on February 21st 1935. He resigned at late 1938, partly as a protest against the small annual funds, which slowed the fortifying works, thus endangering national security.

Only after the political situation worsened in middle Europe, did the fortifying works get more attention from the Finnish general staff. The number of hired men working on different sites before the mobilization (YH in early October) was 600 at most. There were however several sites where, at it's peak, over 3 000 volunteers worked between June 4th and October 10th 1939.
During the period between 1936 - 1939, 38 old concrete positions (bunkers and "concrete trenches") were repaired, 11 old bunkers were modernized (for example, by converting them to flank- firing bunkers) and 52 new ones were battle-ready when the War started.

After the pause, the Inkil�-sector was strengthened and between '32 - '34, the pioneer battalion built 6 bunkers (the 7th was finished in 1937). The bunkers were of relatively good quality and the roof was designed to withstand hits by soviet 152 mm howitzers. The use of armor plates was introduced in the 5 newest bunkers (bunkers Ink 3 -7). The plates (12 plates) were left over from the construction sites of 2 coastal batteries (M�kiluoto and Kuivasaari) and their size was little over 2 metres x 3 metres and thickness either 100mm or 150 mm. The plates enabled the wall, where the firing port was located, to be relatively thin, making it possible to reduce the size of the firing port considerably. When armor plates were used also on the roof, the height of the bunker was reduced by metres making it easier to blend it into the surrounding landscape.

The problem with armor plates was that they were expensive and had to be bought abroad. After the global price of steel rose (as an example, the price of steel plates for three bunkers rose from 3,5 Mio to 5,6 Mio Marks), and the "Casemate de Bourge"-type (shielding the flank-firing mg port by a "wing") bunker building was adopted, the use of armor plates was abandoned. Instead, the compression strength of the concrete, used in bunkers made in '38 and '39, was increased steadily (to about 450 kg/cm�) and sometimes even 600 kg/cm� was achieved. The requirements of a normal bunker was to withstand several hits by 6 inch bombardment, thereby forcing the enemy to use super heavy guns to destroy them.

The Artillery Inspector General V.P.Nenonen was also involved, from the 1920's onward, in the Isthmus fortifying. He had on several occasions asked, what measures were taken to prevent the enemy to drive tanks to block the mg-ports (as indeed often happened during the war) making the bunker harmless. He also pointed out to the designers that the enemy could bring heavy artillery pieces to the frontline and use them to destroy bunkers by direct fire. (Again, he predicted what would happen, as the soviets brought, during the war, in several places heavy 152 mm pieces within 500 metres from individual bunkers and silenced them with accurate shots through the firing port or by slowly wrecking the bunker wall, not designed to take such punishment. Due to shortage of artillery ammunition, the lone guns were not "fat enough" targets and therefore not neutralized by Finnish artillery,)
No solutions were brought up to overcome these possible threats, other than belief in AT-obstacles and infantry AT-teams (funny, since Finland had virtually no AT-weaponry at that time!) and the Finnish artillery.