What son of Grand Duke Lithuania Kęstutis went on to defeat Švitrigaila and his Livonian allies?
Battle of Wilkomierz The Battle of Wilkomierz (see other names) took place on September 1, 1435 near Ukmergė in the Grand Duchy of Lithuania. With the help of military units from the Kingdom of Poland, the forces of Grand Duke Sigismund Kęstutaitis soundly defeated Švitrigaila and his Livonian allies. The battle was a decisive engagement of the Lithuanian Civil War (1432–38). Švitrigaila lost most of his supporters and withdrew to southern Grand Duchy; he was slowly pushed out and eventually made peace. The damage inflicted upon the Livonian Order has been compared to the damage of Battle of Grunwald upon the Teutonic Order. It was fundamentally weakened and ceased to play a major role in Lithuanian affairs. The battle can be seen as the final engagement of the Lithuanian Crusade.
Treaty of Vilnius (1559) The Treaty of Vilnius or Vilna was concluded on 31 August 1559, during the Livonian War, between the Livonian Order and the Polish-Lithuanian Commonwealth at Vilnius (Vilna, Wilna, Wilno). Gotthard von Kettler put the lands of the order under protection of Polish king and Grand Duke of Lithuania, Sigismund II Augustus. The alliance was intended to neutralize the imminent threat of annexation of the order's lands by Russia, yet despite earning military support from Polish-Lithuanian chancellor Mikołaj "the Black" Radziwiłł, Kettler was unable to prevent the occupation of most of Livonia by Russian forces. Thus, Kettler concluded a second treaty with Poland-Lithuania on 28 November 1561, transferring the remnants of the Teutonic Order State to the Polish Crown and the Grand Duchy of Lithuania, himself becoming a vassal of Sigismund II Augustus.