The population of Poland-Lithuania was not overwhelmingly Catholic or Slavic. This circumstance resulted from the federation with Lithuania, where ethnic Poles were a distinct minority. In those days, to be Polish was much less an indication of ethnicity than of rank it was a designation largely reserved for the landed noble class, which included members of Polish and non-Polish origin alike. Generally speaking, the ethnically nonPolish noble families of Lithuania adopted the Polish language and culture. As a result, in the eastern territories of the kingdom a Polish or Polonized aristocracy dominated a peasantry whose great majority was neither Polish nor Catholic. This bred resentment that later grew into separate Lithuanian, Belorussian, and Ukrainian nationalist movements.

In the mid-sixteenth century, Poland-Lithuania sought ways to maintain control of the diverse kingdom in spite of two threatening circumstances. First, since the late 1400s a series of ambitious tsars of the house of Rurik had led Russia in competing with Poland-Lithuania for influence over the Slavic territories located between the two states. Second, Sigismund II Augustus (1548-72) had no male heir. The Jagiellon Dynasty, the strongest link between the halves of the state, would end after his reign. Accordingly, the Union of Lublin of 1569 transformed the loose federation and personal union of the Jagiellonian epoch into the Polish-Lithuanian Commonwealth, deepening and formalizing the bonds between Poland and Lithuania (see fig. 4).

Data as of October 1992