Treaty of Vilnius

…this subsided when, by the Treaty of Vilnius in 1401, Władysław recognized Vytautas as supreme duke of Lithuania on the condition that Poland and Lithuania be indissolubly united by a common foreign policy.

…in a treaty concluded at Vilnius in January 1401. Under the terms of the treaty, the Lithuanian boyars promised that in the event of Vytautas’ death they would recognize Jogaila as grand prince of Lithuania, and the Polish nobility agreed that if Jogaila died they would not elect a new…