In 1385, the Union of Krewo was signed between Queen Jadwiga of Poland and Jogaila, the Grand Duke of Lithuania, the ruler of the last pagan state in Europe. The act arranged for Jogaila's baptism and the couple's marriage, which established the beginning of the Polish–Lithuanian union. After Jogaila's baptism, he was known in Poland by his baptismal name Władysław and the Polish version of his Lithuanian name, Jagiełło. The union strengthened both nations in their shared opposition to the Teutonic Knights and the growing threat of the Grand Duchy of Moscow.
The population of the Grand Duchy's enlarged territory was accordingly heavily Ruthenian and Eastern Orthodox. The territorial expansion led to a confrontation between Lithuania and the Grand Duchy of Moscow, which found itself emerging from the Tatar rule and itself in a process of expansion. Uniquely in Europe, the union connected two states geographically located on the opposite sides of the great civilizational divide between the Western Christian or Latin world, and the Eastern Christian or Byzantine world.