The article, based on the memoirs of P. Bilon, highlights the process of formation and development of the armed forces of the UPR (Ukrainian People's Republic) Directory (1918-1919). Such phenomena as “atamanism”, the unification of the Galician army and the UPR army, and the creation of Ukrainian military aviation are considered. The importance of studying memoirs as a unique source on the history of the Ukrainian revolution is noted. Military memoirs in fact often is the only source for the history of certain military units, conducting combat operations and their direct participants. Considered is the degree of understanding in the memoirs transformational processes of the Ukrainian revolution under the influence of domestic- and foreign-policy situation during of 1918-1919. Lighting of the period of the Ukrainian revolution of 1917-1921 through the prism of memoir literature, comparing it with data from other information sources, and most importantly with official historical doctrine of both the Soviet and modern periods, not only provide a qualitative highlight the main aspects of the research, but also to reveal the "moral feature" of the era. Namely, the mood of specific individuals who have left us memoirs. Thanks to them you can see the many facets of the mentality of that society, which fell into the whirlpool of tumultuous political events. It is noted that the active involvement of the memoir heritage will contribute to a maximum of an objective picture of historical events, to intensify the study of processes in early XX century by representatives of academic historical scholarship.