The first written mention of Cossacks appeared in the late 15th century. The appearance of Cossacks played an outstanding part in the historical fate of Ukraine. Cossacks represented a social standing of free people who defended their land and guarded its boundaries against Turkey-Tatar aggression.
In the middle of the 16th century the Cossacks created their own military-political organization: Zaporizhian Sich. It had the original military-administrative system based on the principles of Cossack democracy. The Cossacks founded specific political institutions such as: institutions of military councils, the Zaporizhian Army Kish as the higher executive-legislative organs, and their own legal proceedings.
The Cossacks were a rather strong and numerous organization. K. Kosynskyi and S. Nalyvaiko led the Cossacks in the first great revolts. They took fortresses, liberated towns and villages, and their law became firmly established in the Kyiv, Volyn and Bratslav provinces. Among other great Cossack leaders were Taras Fedorovych, Pavlo But, Yakiv Ostrainyn, Dmytro Hunia.
In 1633 the Polish government, influenced by the revolts, legalized the existence of the Orthodox Church (of which Petro Mohyla was the Metropolitan), and in 1638 the Warsaw courts abolished the Cossacks' privileges which were previously conquered. Among these Cossack privileges were a legislation of their own, the appointment of officers by election, and the limitation of register.
The defeats of the Cossack rebellions of the 16th to early 17th century made
the grave position of Ukrainians worse. Cossack leaders, as well as thousands
of rebellious Cossacks and peasants, were annihilated and lands were redistributed.
The "gold tranquillity" which prevailed in the early part of the 17th
century proved the tranquillity only for Polish magnates and squires who
had not obtained lessons from the events of the late 16th and early 17th
centuries. Gradual accumulation of force before the decisive conflict
with the powerful state machine of Rich Pospolyta took place in Ukrainian