Excerpts from Books and Wikipedia
"Jogaila, later Wladyslaw II Jagiello (c. 1352/1362 – 1 June 1434) was the Grand Duke of Lithuania (1377–1434) and then the King of Poland (1386–1434), first alongside his wife Jadwiga until 1399, and then sole King of Poland. He ruled in Lithuania from 1377. Born a pagan, in 1386 he converted to Catholicism and was baptized as Wladyslaw in Kraków, married the young Queen Jadwiga, and was crowned King of Poland as Wladyslaw II Jagiello. In 1387 he converted Lithuania to Christianity. His own reign in Poland started in 1399, upon the death of Queen Jadwiga, and lasted a further thirty-five years and laid the foundation for the centuries-long Polish–Lithuanian union." [Wikipedia] "Now, however, the Polish dukes had a proposal for the Grand Duke of Lithuania. If he converted to orthodox Christianity and married Hedwig, he could become king of Poland and Lithuania, a strong country that would be vulnerable to neither Teutonics nor Mongols. The Grand Duke, Jogaila, was in his midtwenties and already showing signs of the political intelligence that would mark the rest of his reign." The Union of Krewo was signed on August 14, 1385.