Poland’s Sigismund II, at war with Tsar Ivan IV of an increasingly powerful Russian state over Livonia, which had been absorbed into Poland-Lithuania, undertakes the formal consolidation of the two nations, loosely federated for two centuries.

Adroitly playing off the Polish gentry against the Lithuanian magnates, he secures the Union of Lublin in 1569, by which the Kingdom of Poland and the Grand Duchy of Lithuania, together with Livonia and Ducal Prussia, formally merge in the Polish-Lithuanian Commonwealth, with one elective monarch and a common parliament.

Sigismund also devises a system of checks and balances between the crown and parliament.

Lithuania, however, loses its separate institutions under the reorganized federation.

The Polish-Lithuanian Commonwealth now controls territories from the Baltic to the Black seas.

Under the union, most Ukrainian lands are transferred to the direct jurisdiction of the Polish crown.