The first volume of this study covers the period from the end of July 1914, until the transition of Bulgaria to the side of the central powers. The work was written on the basis of the study of primary sources: Russian, German, English, Austrian, French and Italian published archival documents related to the July crisis of 1914; Russian archival documents for 1914-1915; foreign memoir literature; parliamentary reports; Russian and foreign periodicals; stenographic protocols of the commissions of the German National Assembly and the German Reichstag, protocols of the Extraordinary Commission of Inquiry of the Russian Provisional Government, containing interrogations of former ministers, ambassadors, commanders-in-chief, statesmen and public figures and other persons who directly participated in the events. Foreign monographic literature was also used. This work was the first to use Russian archival documents published in the series "International Relations in the Era of Imperialism"; until now, they have been stubbornly hushed up by foreign historians.