 Gediminas daughter Aldona married Władysław Łokietek's son and future successor, Casimir III of Poland, in 1325, which led to improved relations.In 1358 a treaty between Duchy of Masovia and Grand Duchy of Lithuania outlined a border between the two for the first time.In 1385 the growing threat of the Teutonic Order to both countries led to a firmer alliance, the Union of Krewo, which signaled the beginning of a centuries-long Polish–Lithuanian union.