[5] Towards the end of the 15th century, the Ukrainian Cossacks formed the Zaporozhian Sich centered on the fortified Dnipro islands.
[1] While the Cossacks were useful to the Polish-Lithuanian states in the war periods, they proved to be more problematic in the peacetime, due to their raids on the Commonwealth neighbours (primarily, the Ottoman Empire and its allies).
[1] Afterward the Khmelnytsky Uprising, the Treaty of Pereyaslav brought most of the Cossack Hetmanate under Russian control.
[9] The Cossacks provided refuge for runaway serfs and bandits, and often mounted unauthorized raids and pirate expeditions against the Ottoman Empire.
[9] As the empire attempted to limit Cossack autonomy in the 17th and 18th centuries, this resulted in rebellions, such as those led by Stenka Razin, Kondraty Bulavin and Yemelyan Pugachev.