For the first time, the reader is offered a special edition, which comprehensively tells about the Armed Forces of the Ukrainian state during the time of Bogdan Khmelnitsky, in particular about the structure of the army, its strategy and tactics, the manufacture of weapons, intelligence, diplomatic affairs, the organization of border guards, etc. Including the testimonies of outstanding military specialists, primarily contemporaries of the era of Bogdan Khmelnitsky, including foreign ones, the book reasonably proves that the Cossack army in many respects significantly exceeded the armies of the most developed European countries.