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Why did Sigismund II Augustus want a stronger union between Poland and Lithuania?

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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.

Sigismund II Augustus sought a stronger union between Poland and Lithuania primarily to secure the future of the state and address pressing military threats. As the last king of the Jagiellon dynasty, Sigismund II had no heirs, which created a significant risk that the personal union between the two countries would dissolve upon his death.123

Furthermore, the Livonian War, which began in 1558, presented an urgent need for a unified defense against Muscovy. Because Lithuania could not bear the heavy financial and military burden of this conflict alone, the king aimed to create a more robust, constitutionally unified state to better manage foreign policy and military resources.13

While some powerful Lithuanian magnates initially resisted the merger out of fear that it would diminish their influence, the king successfully leveraged the support of the gentry—who desired the privileges held by the Polish szlachta—and used territorial annexation to force negotiations. These efforts culminated in the Union of Lublin in 1569, which established the Polish-Lithuanian Commonwealth as a federated state.134