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How did the Livonian War increase pressure for a 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.

The Livonian War (1558–1583) acted as a primary catalyst for the Union of Lublin by exposing the vulnerability of the Grand Duchy of Lithuania to Muscovite expansion.12

The war placed a severe strain on Lithuania’s resources, forcing the state to seek military assistance from Poland. This necessity shifted the political landscape in several ways:1

Security Concerns: The serious threat posed by Ivan the Terrible convinced many members of the Lithuanian gentry that a closer union with Poland was essential for survival, leading them to propose a merger as early as 1562.23
Political Leverage: While powerful Lithuanian magnates initially blocked these proposals to protect their own authority, the king, Sigismund II Augustus, used the crisis to force the issue. By annexing significant Lithuanian territories—including Volhynia and the Kyiv region—into Poland, he compelled the Lithuanian leadership to negotiate.123
Consolidation: Facing the prospect of further territorial loss, the Lithuanian opposition ultimately conceded, resulting in the 1569 union that created the Polish-Lithuanian Commonwealth, a federated state designed to better defend against eastern threats.234