The memoirs of Colonel D. L. Kazantsev cover the period 1914–1917, when he served in the Operations Office in Finland. The publication of this source opens a virtually unknown front of the First World War! war, where there was also confrontation between the warring parties. The author pays great attention to the development of the revolutionary activist (jaeger) movement in Finland, the process and methods of forming the Royal Prussian Jaeger Battalion No. 27 in the German army from the Finns, as well as the struggle of the Russian authorities with the activist movement and recruitment into the German army. The memoirs cover almost the entire period of the First World War and end with a description of the revolutionary events in Helsingfors, the massacres of Russian officers and the formation of councils of workers' and soldiers' deputies.