Registered Cossacks

Zaporozhian Host
Active	1572 - 1648
Country	Polish-Lithuanian Commonwealth
Allegiance	Poland
Branch	land forces
Role	infantry (primarily)/ cavalry
Size	up to 40,000 (1649)
Garrison/HQ	Trakhtemyriv, Kiev Voivodeship
Engagements	Livonian War, Moldavian Magnate Wars, Polish–Ottoman War, Polish–Muscovite War
Commanders
Notable commanders	Petro Sahaidachny, Mykhailo Doroshenko, others

Registered Cossacks (Ukrainian: Реєстрові козаки, Rejestrovi kozaky, Polish: Kozacy rejestrowi) comprised special Cossack units of the Polish–Lithuanian Commonwealth army in the 16th and 17th centuries.

Registered Cossacks became a military formation of the Commonwealth army beginning in 1572 soon after the Union of Lublin (1569), when most of the territory of modern Ukraine passed to the Crown of Poland. Registered Cossack formations were based on the Zaporozhian Cossacks who already lived on the lower reaches of the Dnieper River amidst the Pontic steppes as well as on self-defense formations within settlements in the region of modern Central and Southern Ukraine.

Historically, the first official plan for the creation of Cossack formations as a border service was brought to the State Council of the Grand Duchy of Lithuania in 1524 by Semen Polozovic and Kristof Kmitic. However, due to the lack of funds, the idea was not realized. The starosta of Cherkasy, Ostap Dashkevych, revived the idea at the 1533 council in Piotrków Trybunalski. Dashkevych tried to show that in order to protect the borders beyond the Dnieper it would be necessary to maintain an army of 2,000 soldiers and several hundred cavalrymen. He pointed out the importance of establishing forts on the river's islands to keep Tatar raids in check.

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Wikipedia