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Why did Poland and Lithuania form the Union of Lublin?

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Map showing Poland's territory in 1634 under the reign of Wladyslaw IV Vasa, shaded in pink and spanning from the Baltic Sea in the north to near Kyiv in the southeast. Major present-day cities such as Warsaw, Krakow, Gdansk, Vilnius, Minsk, Lwow (Lviv), Pinsk, and Kyiv are marked within the territory, with surrounding cities like Moscow, Riga, Vienna, and Budapest shown for reference. — Polish-Lithuanian Commonwealth Poland's territory in 1634, during the reign of Władysław IV Vasa.

The Union of Lublin (1569) was formed to transform the personal union between Poland and Lithuania into a single, permanent, and indivisible state. Several key factors drove this consolidation:12

Succession Concerns: King Sigismund II Augustus had no heirs. The Polish nobility feared that upon his death, the personal union—which had existed since 1385—would dissolve, potentially separating the two realms.12
External Threats: The Livonian War against Muscovy exposed the vulnerability of the Lithuanian frontier. Many members of the Lithuanian gentry sought a closer union with Poland to better secure their defense against Ivan the Terrible.12
Political Maneuvering: While Lithuanian magnates initially resisted the merger to protect their own power, Sigismund II Augustus forced the issue by annexing significant Lithuanian territories (including Volhynia and Kiev) into the Polish kingdom. Faced with the loss of these lands, the remaining Lithuanian representatives agreed to the union to preserve their state's integrity within a federated structure.12

This pact created a unified body politic that remained a major European power until the late 18th century.12