Soviet historiography has repeatedly addressed to the theme of the participation of the French Left in the revolution of 1917, and in particular to the French military mission’s officer, Captain Jacques Sadoul. It is considered that he was an active fighter for the revolution and a devoted friend of the Soviet Republic. A unique source – «The Dossier of the French Communist Jacques Sadoul», which kept in the archive of the Historical Service of the French Ministry of Defense allows us to look at the personality of the famous French socialist differently and clarify some unknown aspects of his activity. The authors represent Sadoul as a political mediator, demanded by the Soviet and the French governments. It was especially evident in 1917-1918, when Jacques Sadoul took an active part in project of the Red Army’s reorganization and decided the fate of the allied intervention in Russia. The article pays attention to the conflicting views of compatriots about Sadoul, and also to his activity in the 1919-1920s: it seems likely that Captain Sadoul was an unofficial agent of the French Left.