Finland is one of the world’s most northern and geographically remote countries and is subject to a severe climate. Nearly two-thirds of Finland is blanketed by thick woodlands, making it the most densely forested country in Europe. A part of Sweden from the 12th century until 1809, Finland was then a Russian grand duchy until, following the Russian Revolution, the Finns declared independence on December 6, 1917. Throughout the Cold War era, Finland maintained a neutral political position, although a 1948 treaty with the Soviet Union (terminated in 1991) required Finland to repel any attack on the Soviet Union carried out through Finnish territory by Germany or any of its allies. Finland was admitted to the United Nations in 1955.