After being split up for a long time, Poland was unified by King Władysław at the beginning of the fourteenth century. His successor, Casimir III, led the country to the height of its prosperity thanks to his clever peace-oriented diplomacy. Casimir III passed away without having an heir and Jadwiga ascended the throne after a series of events. However, Poland sought a strong leader due to increased pressure from the Teutonic Knights. So, because it was in their common interest, Jogaila, the Grand Duke of Lithuania, and Jadwiga got married, and so, Poland and Lithuania formed an alliance.