Registered Cossacks (Ukrainian: Реєстрові козаки, Reyestrovi kozaky, Polish: Kozacy rejestrowi) is the term used for Ukrainian Cossacks (mostly from the Zaporizhian Sich) who were part of the Polish-Lithuanian Commonwealth armies. Registered Cossacks were a part of Commonwealth army from 1582 until the year 1699. The idea came from Polish king Stefan Batory and the first register had 500 names.
Registered Cossacks formed an elite among Cossacks, serving in the military under commanders (starshyna) and main otaman, who were responsible before Grand Crown Hetman (Commonwealth highest military commander). A substantial percent of Cossacks formed skilled light cavalry units (choragiew), excellent skirmishers trained in mounted archery (and later using firearms), making lightning raids, harassing heavier, slower formations and disengaging. Those units were often used as support for heavy elite Commonwealth cavalry, the hussars, and were much cheaper to form than a hussar unit. Cossack units were also known for their tabor formation.
Registered Cossacks had many privileges, including personal freedom, exclusion from many taxes and duties, and the right to receive wages (although the Commonwealth military was plagued with fiscal problems, leading to extremely delayed wages, often paid in items like clothes or weapons instead of coin).
Many Cossacks were skilled warriors, and Cossacks' major income source came from raids on the southern neighbors of the Commonwealth (Ottoman Empire and its vassals). However only a small number were actually 'registered Cossacks' - the exact number was from few hundred to few thousands and varied in time, usually being increased during wartime. This has led to many social and political tensions, especially as szlachta (Polish and Ukrainian gentry) continually attempted to force Cossacks into submissions as peasants, while Cossacks demanded the significant expansion of the Cossack register. Furthermore, the Cossack-szlachta conflict was aggravated as Cossacks often supported Commonwealth monarchs (like Wladyslaw IV Waza), who were often at odds with Polish szlachta, wishing to further limit the monarch's powers. The tensions between Cossacks and Polish szlachta grew and from the late 16th century resulted in several uprisings (the greatest of which was the Khmelnytsky uprising of 1648), with registered Cossacks often forced to choose sides between supporting their own people or the szlachta-backed Commonwealth forces.
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