early age. Before that, he attended a Jesuit college, either in Kraków or Sandomierz, achieving a solid secondary education. After finishing his schooling, he became a courtier to then-royal prince and future king of Poland John II Casimir Vasa. As Stefan could not afford the equipment needed to join a hussar unit, he learned the art of war serving with the Lisowczycy mercenaries, joining them as towarzysz (companion, a junior cavalry officer) in the spring of 1621. Already an officer at the age of eighteen, he took part in the battle of Chocim (Khotyn) in 1621, where the Commonwealth army stopped the Ottomans and ended the Polish–Ottoman War (1620–21). His years with the brutal Lisowczycy mercenaries would impact his strategy in the latter years, where he would not hesitate to use any means necessary – including mistreatment of civilians – to achieve victory. The next time he took arms was in 1623, when his brother Paweł received a nomination as a rotmistrz of light (cossack) cavalry of the regular army (wojsko kwarciane), and recruited some of his siblings, including Stefan, into his unit. They served under hetman Stanisław Koniecpolski in the campaigns against the Tatars in 1624 (participating in the battle of Martynów). In 1625 they took part in the quelling of a Cossack Zhmaylo Uprising. He fought later against Gustavus Adolphus (1626–1629) (part of the Polish–Swedish War). His light cavalry was used in reconnaissance, diversion, and raiding the enemy territory. In 1627 Stefan, serving in his brother's chorągiew, was promoted to chorąży. On 6–7 August that year he participated in the battle of Tczew, where Gustavus Adolphus was nearly killed. During those years, he learned much from observing a master of military arts, hetman Koniecpolski, who often commanded the Polish army in which he served, and from Poland's Swedish opponents, and earlier from the Cossacks and Tatars they fought. As the Polish–Swedish war ended, in 1630, Czarniecki enlisted in the Habsburg forces and continued to fight against the Swedes, participating in the battle of Breitenfeld. In 1633 he joined the Commonwealth forces again, to fight under the new Polish king, Władysław IV, in the Smolensk War against Muscovy that lasted until 1634. During the Smolensk War he learned Western tactics used by the foreign mercenary troops. In December 1633 he commanded a raiding unit that successfully raided and pillaged behind the Russian lines, spreading terror, disrupting supply lines, and