The article is devoted to a comprehensive study of Soviet military assistance to Republican Spain in the Civil War of 1936-1939. The paper reveals the decision-making processes of the Soviet government to provide military provision to Spain, the implementation of operation "X". The activities of military advisers in Spain and the legal framework of their activities are revealed. The military participation of volunteers and their exploits are also considered. The article draws conclusions and summarizes the provision of military assistance to Spain for the USSR.