The monograph examines events previously unexplored by Russian historians related to Finland’s entry into the Second World War on the side of Germany. Hidden preparations for this in 1940–1941. is revealed on the basis of Finnish, German and Russian archives, as well as other sources. The book traces the stages of rapprochement in military, economic and political relations between Finland and Germany on the eve of the Great Patriotic War. The changes in the position of the USSR towards Finland in the pre-war period are especially studied. For historians and anyone interested in the history of Finland and the Second World War.