Lake Ladoga, in northern Russia, is the largest lake in Europe, having a surface area of 17,891 square kilometres. It occupies a strategic position north and east of Saint Petersburg, and between 1918 and 1940 formed part of the border between Finland and the Soviet Union. In the Winter War of 1939-40 Finnish troops inflicted severe reverses on invading Soviet forces in the Karelian Isthmus - the strip of land between the lake and the Gulf of Finland - before being overwhelmed by vastly superior numbers; and in the Continuation War on 1941-44 they again occupied the isthmus, before being forced to make peace with the Soviets. German troops also reached the shores of Lake Ladoga from the south west during the Siege of Leningrad. During the 900-day siege, the city could only be supplied by boat (or in winter by trucks crossing the ice) across the lake.