From Finland’s War of Choice: The Troubled German-Finnish Coalition in World War II, by Henrik Lunde (Casemate, 2011), Kindle Loc. 348-56, 406-30:
The Soviet Union attacked Finland on November 30, 1939, hoping for a quick victory. However, the attack bogged down with the Soviets suffering heavy losses. After regrouping and bringing up reinforcements, the Soviets resumed their offensive on February 1, 1940. It was to last for forty-two days. The Soviet attack on the Karelian Isthmus was backed by thirty infantry divisions reinforced by strong artillery and armored forces. After two weeks of ferocious fighting resulting in enormous Soviet casualties, the Mannerheim Line was breached on February 13 and by March 1 the Finnish right flank had been pushed back to the city of Viipuri. The situation for the Finns had become desperate. They were short of supplies and their troops were exhausted. The hoped-for—and promised—assistance from the West had not materialized. The total number of foreign volunteers in Finland numbered only 11,500 and 8,275 of these were from Scandinavia—mostly from Sweden. The volunteers also included 300 men in the Finnish-American Legion who received their baptism of fire in the last days of the war.
While the Soviet losses in the Winter War have never been published, most observers believe that more than 200,000 were killed and a much larger number wounded. The Finns lost 24,923 killed and 43,557 wounded. This was an enormous loss for a nation with a population of only 3.75 million.
The territorial losses resulting from the Winter War amounted to about 64,750 square kilometers or about 10 per cent of Finland’s total prewar area, containing about 12 per cent of the population. The Karelian Isthmus, including the province and city of Viipuri, and a large piece of territory north of Lake Ladoga were lost. The loss in resources and manufacturing capacity was devastating. The losses in agricultural lands, forestry, and production of forestry products were almost as severe.
Also lost were several islands in the Gulf of Finland, part of the Rybachiy Peninsula in the far north, and large segments in the Salla-Kuusamo area in the central part of the country. Finland was forced to lease Hanko and the surrounding area at the entrance to the Gulf of Finland to the Soviets for a period of 30 years. Hanko, along with Viipuri, had handled about a quarter of all Finnish exports.
Finland also had to agree to extend the railway from Kemijärvi (southwest of Salla) to the new frontier at Salla within a year. The Pechenga area which had been occupied by the Russians was returned to Finland, probably because of the foreign interests in the nickel mines.
The war left Finland with a monumental problem of having to move almost the entire population—between 400,000 and 500,000 people—of the lost territories to other parts of the country. While these included skilled and semi-skilled workers, a large portion consisted of independent farmers. The resettlement operation, which created new homesteads for the displaced farmers, also produced internal tensions. Much of the land on which these refugees were resettled was in the Swedish-speaking area of the country and this caused some difficult situations.
Finally, the ceded territories represented a crushing strategic blow as they “left the country” in the words of Mannerheim “open to attack and the Hanko base was like a pistol aimed at the heart of the country and its most important communications.” The border on the Karelian Isthmus and in the Lake Ladoga area was pushed back and had no fortifications. The war had demonstrated that the Finns did not have the manpower to adequately defend the central and northern area of the country. Acquisition of the Salla area and the demand that the Finns construct a railway from Kemijärvi to Salla where it would connect with a line being constructed by the Soviets was alarming. It created an opportunity for the Soviets to quickly penetrate the waist of Finland to the Swedish border.