Hetmans of the Polish–Lithuanian Commonwealth in the context of Stefan Czarniecki

Konstanty Iwanowicz Ostrogski (c. 1460 – 10 August 1530) was a Ruthenian prince and magnate of the Grand Duchy of Lithuania. He later had the title of grand hetman of Lithuania from 11 September 1497 until his death in 1530.

The military of the Polish–Lithuanian Commonwealth consisted of two separate armies of the Kingdom of Poland's Crown Army and the Grand Duchy of Lithuania's Grand Ducal Lithuanian Army following the 1569 Union of Lublin, which joined to form the bi-conderate elective monarchy of the Polish–Lithuanian Commonwealth. The army of each country was commanded by their respective Hetmans. A distinctive formation of both armies were the Winged hussars. The Polish–Lithuanian Commonwealth Navy never played a major role and ceased to exist in 1643.

Commonwealth forces were engaged in numerous conflicts in the south (against the Ottoman Empire), the east (against the Tsardom of Muscovy and later, the Russian Empire) and the north (the Kingdom of Sweden); as well as internal conflicts (most notably, numerous Cossack uprisings). For the first century or so, the Commonwealth military was usually successful, but became less so from around the mid-17th century. Plagued by insufficient funds, it found itself increasingly hard-pressed to defend the country, and inferior in numbers to the growing armies of the Commonwealth's neighbors.

The Battle of Berestechko (Ukrainian: Битва під Берестечком, Polish: Bitwa pod Beresteczkiem; 28 June – 10 July 1651) was fought between the Cossack Hetmanate and Crimean Khanate against the Polish–Lithuanian Commonwealth as a part of the Khmelnytsky Uprising. Near the site of the present-day city of Berestechko in Ukraine forces of the Zaporozhian Cossacks and Crimean Tatars under the command of Hetman Bohdan Khmelnytsky, Otaman Tymofiy Khmelnytsky, Colonels Ivan Bohun and Fylon Dzhalaliy with Khan İslâm III Giray and Tugay Bey, who was killed in the battle, were defeated by the Polish–Lithuanian Commonwealth's forces under the command of the Polish King John II Casimir, Prince Jeremi Wiśniowiecki, Hetmans Marcin Kalinowski and Stanisław Lanckoroński. The battle took place in the Volhynian Voivodeship on a hilly plain south of the Styr River. The Polish–Lithuanian camp was on the Styr River opposite Berestechko and faced south, towards the Zaporozhian Cossack positions about two kilometers away, with the right flank of the latter covered by the Pliashivka (Pliashova) River and the left flank by Crimean Tatars. It is considered to have been among the largest European land battles of the 17th century.