From Wikipedia King Władysław II Jagiełło of Poland: (ca. 1362 – 1 June 1434) was Grand Duke of Lithuania (1377–1434), king-consort of Kingdom of Poland (1386–1399), and sole King of Poland (1399–1434). He ruled in Lithuania from 1377, at first with his uncle Kęstutis. In 1386, he converted Lithuania to Christianity, was baptized as Władysław, married the young queen regnant Jadwiga of Poland, and was crowned Poland's king as Władysław Jagiełło. His own reign in Poland started in 1399, upon death of Queen Jadwiga, and lasted a further thirty-five years and laid the foundation for the centuries-long Polish–Lithuanian union. Władysław II was the founder of the new Jagiellon dynasty that bears his name, while pagan Jogaila was an heir to the already established house of Gediminids (Gediminid dynasty) in Grand Duchy of Lithuania; his royal dynasty ruled both states until 1572, and became one of the most influential dynasties in the late medieval and early modern medieval Central and Eastern Europe.
Jogaila was the last pagan ruler of medieval Lithuania. He held the title Didysis Kunigaikštis. As King of Poland, he pursued a policy of close alliances with Lithuania against the Teutonic Knights. The allied victory at the Battle of Grunwald in 1410, followed by the Peace of Thorn (1411), secured the Polish and Lithuanian borders and marked the emergence of the Polish–Lithuanian alliance as a significant force in Europe. The reign of Władysław II Jagiełło extended Polish frontiers and is often considered the beginning of Poland's "Golden Age".