How did the Union of Lublin change the relationship between Poland and Lithuania?

The Union of Lublin (1569) fundamentally transformed the relationship between Poland and Lithuania by replacing their previous personal union under a single sovereign with a formal, federated state known as the Polish-Lithuanian Commonwealth.

While the agreement stipulated that both nations were to remain distinct, equal components—each retaining its own army, treasury, laws, and administration—the reality shifted toward Polish dominance. This imbalance was cemented when King Sigismund II Augustus annexed over one-third of Lithuania’s territory, including Volhynia and the regions of Kiev, into Poland prior to the final agreement.

Consequently, Poland held greater representation in the joint Sejm, and Lithuania increasingly became the subordinate partner. Over time, this led to the intense Polonization of the Lithuanian nobility and the adoption of Polish political and cultural models, with the Polish language eventually replacing Ruthenian as the official language for administration and law.12