The first volume of the publication is a documentary prehistory of the Russian-Turkish war of 1877-1878. It includes 460 documents (78 for 1875, 322 for 1876, 60 for 1877), of which 26 have already been previously published in other publications. The chronological framework of the first volume is from the beginning of the uprising in Herzegovina in the summer of 1875 to April 12 (24), 1877, the day of the declaration of war. To prepare the first volume of the publication, the project participants examined the materials of 72 archival funds in the repositories of the USSR and over 60 funds in Bulgaria. The main theme of the volume is public relations between Russia and the southern Slavs. Showing the attitude of Russian society to modern events on the Balkan Peninsula and Russia's ties with the southern Slavs, the volume's material displays both events in Bulgaria, Serbia, Montenegro, Bosnia and Herzegovina, and the activities of Bulgarian, Serbian and other public organizations.