The Union of Krewo was a significant agreement made in 1385 between the Grand Duchy of Lithuania and the Kingdom of Poland. This union aimed to strengthen both states against external threats, particularly from the Teutonic Knights. It marked the beginning of a long-lasting political alliance that would eventually lead to the formation of the Polish-Lithuanian Commonwealth. The union was initiated by the marriage of Jadwiga, the Queen of Poland, and Władysław II Jagiełło, the Grand Duke of Lithuania. This alliance not only united the two nations politically but also facilitated the spread of Christianity in Lithuania, as Jagiełło converted to Catholicism. The Union of Krewo laid the groundwork for future cooperation and integration between Poland and Lithuania.