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How did Jogaila become king of Poland?

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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.

Jogaila, the Grand Duke of Lithuania, became the King of Poland through a strategic political and marital alliance finalized by the Union of Krewo in 1385.12

Polish lords sought to strengthen their kingdom against the Teutonic Knights and expand their influence by uniting with the vast territories of Lithuania. To secure the throne, Jogaila agreed to several conditions: he converted to Roman Catholicism, committed to introducing Christianity to his Lithuanian subjects, and pledged to join his Lithuanian and Ruthenian lands to the Polish crown.123

Following these negotiations, Jogaila was baptized in Kraków in February 1386, married the Polish queen Jadwiga, and was crowned as Władysław II Jagiełło on March 4, 1386. Initially, he and Jadwiga ruled the kingdom jointly. This union fundamentally altered the balance of power in Central Europe and established the foundation for the long-standing partnership between the two nations.412