Bohdan Khmelnytsky’s uprising was a turning point in the history of the Polish-Lithuanian Common-wealth, and historians appropriately consider 1648 the point that separates its glory days from its decline. The Commonwealth’s losses at Korsun, Pyliavtsi, Batih etc. and their de facto ‘capitulations’ at Zboriv and Zhvanetz proved not only the crisis of the old Polish military art but also how complicated the socio-economic situation in the Polish-Lithuanian Commonwealth had become. Attempts to solve the ‘Cossack question’ in a peaceful way were neither far-sighted nor brave enough. At the same time, attempts to reform the state failed because of the Civil war (Lubomirski’s rokosz) of 1665–1666. Cossack Ukraine (Hetmanshchyna) was also going through difficult times: de facto independence from the Commonwealth that they achieved in 1649 was nominally lost in 5 years. After a short-lived upturn, lack of reforms, which were practically impossible under the conditions of the never-ending wars, and lack of will on behalf of Hetmans that followed Khmelnytsky, led to the quick stagnation of the Cossack military art. The main purpose of this article was to demonstrate the evolution of the military art of Bohdan Khmelnytsky and the other Cossack colonels, as well as to analyse their successes and mistakes during 1648–1651. Additionally, the reasons behind the great Cossack-Tatar victories during the first years of the uprising of Bohdan Khmelnytsky were outlined. At the end I explain why the great victories of 1648–1651, as well as the whole complex of socio-economic-political changes that took place on the territory of modern Ukraine since the mid 17th century ended in a de facto defeat. Article received 10.03.2019