The Mannerheim Line was a defensive fortification line on the Karelian Isthmus built by Finland against the Soviet Union. While this was never an officially designated name, during the Winter War it became known as the Mannerheim Line, after Finnish Army's then commander-in-chief Field Marshal Baron Carl Gustaf Emil Mannerheim. The line was constructed in two phases: 1920–1924 and 1932–1939. By November 1939, when the Winter War began, the line was by no means complete.