Polish-Lithuanian Commonwealth (1300–1600)
On the Ukrainian territory, the state of Kievan Rus' was succeeded by the principalities of Halych and Volodymyr-Volynskyi, which were merged into the state of Halych-Volynia. In the mid-fourteenth century it was subjugated by Casimir IV of Poland while the heartland of Rus', including Kiev, fell under the Gediminids of Grand Duchy of Lithuania. Following the 1386 marriage of Lithuania's Grand Duke Jagiello to Poland's King Jadwiga (her title was "King" even though she was a woman), most of the Ukrainian territory was controlled by the increasingly Ruthenized Lithuanian rulers as part of the Grand Duchy of Lithuania (the term Ruthenia and Ruthenians as the Latinized versions of "Rus'", became widely applied to the land and its people, respectively).
By the 1569 Union of Lublin that formed the Polish-Lithuanian Commonwealth, a significant part of Ukrainian territory was moved from largely Ruthenized Lithuanian rule to the Polish administration, as it was transferred to the Polish Crown. Under the cultural pressure of polonization much of the Ruthenian upper class converted to Catholicism (such transitions were beneficial for achieving political influence within the state), for example, King Michael of Poland, who reigned from 1669 to 1673, was of the Ruthenian Vishnevetsky Wiśniowiecki family. At the same time the common people, especially the peasants retained their old ways of especially, the allegiance to their historic Eastern Orthodox Church, which led to the increasing social tensions, visible in such events as the 1596 Union of Brest, created by Sigismund III Vasa, who attempted to bring the Orthodox population under the Catholicism through creation of the Ukrainian Greek Catholic Church. This controversial move failed to achieve its goals. Resisted even by some Ruthenian magnates, otherwise loyal to the Polish kings (Ostrogskis being the most notable example), the new "intermediate" religion was unnecessary for the most of the upper class, much of whom increasingly turned directly towards Catholicism with each subsequent generation. Thus, the Ukrainian commoners, deprived of their native protectors among Ruthenian nobility, turned for protection to the Cossacks who remained fiercely Orthodox at all times.
