How was the Polish-Lithuanian Commonwealth created in 1569?

The Polish-Lithuanian Commonwealth was established on July 1, 1569, through the Union of Lublin. This pact transformed the existing personal union between Poland and Lithuania—which had shared a sovereign since 1385—into a single, federated state.1

The move toward a more complete union was driven by several factors:
* Succession Concerns: King Sigismund II Augustus lacked heirs, and the Polish gentry feared the personal union would dissolve upon his death.12
* Military Necessity: The Livonian War and the threat from Muscovy encouraged the Lithuanian gentry to seek closer ties with Poland for protection.12
* Political Pressure: When negotiations stalled, Sigismund II annexed significant Lithuanian territories, including Podlasie and Volhynia, forcing the Lithuanian magnates to resume talks.12

The resulting Polish-Lithuanian Commonwealth was ruled by a jointly selected sovereign. While both nations remained distinct with their own armies, treasuries, and laws, they participated in a joint Sejm and coordinated their foreign policy.1