Karelian Isthmus, land bridge, NW European Russia, connecting Russia and Finland. Situated between the Gulf of Finland in the west and Lake Ladoga in the east, it is 25 to 70 mi (40–113 km) wide and 90 mi (145 km) long. Saint Petersburg and Vyborg (Viipuri) are its chief cities. Originally part of the Grand Duchy of Sweden, the isthmus passed to Russia in 1721, and—except for its southernmost section—became part of Finland in 1917. The Mannerheim Line, which crossed the isthmus, was breached in 1940 by the Russians, who occupied the area. It was briefly held (1941–44) by Finnish and German units during World War II. The isthmus was formally ceded to the USSR in 1944, and more than 400,000 of its Finnish residents moved into Finland.