Editing Mannerheim Line (section)

==Soviet intelligence==
[[File:Trenches mannerheim line winter war.png|thumb|Infantry in a trench on the Mannerheim Line]]

[[Soviet intelligence]] worked in Finland on multiple levels. The Finnish communist party, run from the Soviet Union, 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 [[Military counterintelligence of the Soviet Army|the Fourth Department of the Army General Staff]]. [[Leningrad Military District]], the [[Baltic Fleet]] and border troops under the NKVD conducted [[espionage]] operations.<ref name="UittoGeust3">[[#UittoGeust2006|Geust & Uitto 2006]], pages 15–16</ref>

The Finns exposed two espionage cases during the 1930s. Vilho Pentikäinen, a photographer serving on the Finnish general Staff, escaped to the Soviet Union in 1933. The second case was of Simo Haukka; he took photographs and measured roads and terrain for Soviet intelligence in 1935.<ref name="UittoGeust3" />

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.<ref name="UittoGeust3" /> 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 Red Army officers. It was called "''Finland. Written Description of March Routes''". It was later translated and republished as the "''Red Army March Guide to Finland''". The guide included over 200 pages of maps and photographs.<ref name="UittoGeust4">[[#UittoGeust2006|Geust & Uitto 2006]], pages 36–37</ref>

Along with the intelligence, the Soviet Union received a detailed map of the defences on the Isthmus. A German [[military attaché]] in [[Helsinki]], General Arniké, handed it over in Moscow in September 1939.<ref name="Edwards13" />