In 1944, the Soviet troops were able to break through the Finnish fortifications in a week, and on June 20 to seize Vyborg in their attack on the Karelian Isthmus. Nevertheless, it is not clear what role, when organizing the reflection of the Soviet offensive, was played by Marshal K. G. E. Mannerheim.. An analysis of the sources and literature shows that the inability of the Finnish troops to repel the Soviet offensive is explained by the views of Mannerheim. Marshal made a clear mistake here. The reason of this error was his subjective political views. As a result, Mannerheim could not foresee the scale of the Soviet offensive. In this respect, Prime Minister E. Lincomies was right. He pointed out in his memoirs that Mannerheim «was not an infallible genius … He was a person, but he was also an old and very tired person who also made mistakes…».