Abstract: The subject of this research is the consideration of military operations in Transbaikal territory at the finals stage of Civil War and foreign military intervention (February – November of 1920) with participation of the Orenburg Cossack military detachments. The selected framework allowed conducting a meticulous analysis of events of the past, as well as the entire complex of the major driving forces and factors. Leaning on remembrances of the participants of military operation, archival documents and scientific works, an attempt is made to trace the battle rout of the Orenburg Cossack detachments in Transbaikal, where they arrived in February of 1920 with the remnants of the Admiral Kolchak’s armies. The article explores the reorganization and participation of military units of the Orenburg Cossacks as a part of Far Eastern Army in battles against the People’s Revolutionary Army of the Far Eastern Republic and partisan regiments of the red forces in Transbaikal territory. The scientific novelty lies in generalization of information from the various published and previously unknown sources on the little-studied, but exceptionally dramatic pages of the history of Civil War and foreign military intervention in Transbaikal territory, which is relevant from the perspective of restoring a full picture of the events.