thumb|left|Monument at the field of the Battle of Wiłkomierz Švitrigaila's forces, which included early Cossack, sought external support from the Teutonic Order, which had its own interests in weakening both Poland and Lithuania. The Teutonic Order's involvement further complicated the political situation. However, Sigismund Kęstutaitis found strong backing from Polish King Władysław II Jagiełło (Jogaila), as well as from the majority of the Lithuanian nobility. This external support proved pivotal in the conflict, tipping the scales in Sigismund's favor. The turning point in the war came with the Battle of Wiłkomierz (modern-day Ukmergė, Lithuania) on September 1, 1435. This battle was one of the largest and most decisive military engagements in Eastern Europe during the 15th century. Švitrigaila's forces were decisively defeated, suffering a crushing blow that effectively ended the Grand Principality of Rus'. The defeat led to the collapse of Švitrigaila's short-lived state and ensured the continuation of Lithuanian rule over the Rus' lands. With the victory, Sigismund Kęstutaitis solidified his power as Grand Duke, and the Rus' territories were once again integrated into the Grand Duchy of Lithuania. However, these lands would now be firmly under Lithuanian control, with no significant autonomy for the Rus' people. The failure of the Grand Principality of Rus' also marked a significant shift in the governance of the region, as the aspirations of the Rus' nobility for greater independence were suppressed.
thumb|left|Monument at the field of the Battle of Wiłkomierz Švitrigaila's forces, which included early Cossack, sought external support from the Teutonic Order which had its own interests in weakening both Poland and Lithuania. The Teutonic Order's involvement further complicated the political situation. However, Sigismund Kęstutaitis found strong backing from Polish King Władysław II Jagiełło (Jogaila), as well as from the majority of the Lithuanian nobility. This external support proved pivotal in the conflict, tipping the scales in Sigismund's favor. The turning point in the war came with the Battle o,f Wiłkomierz (modern-day Ukmergė, Lithuania) on September 1 1435. This battle was one of the largest and most decisive military engagements in Eastern Europe during the 15th century. Švitrigaila's forces were decisively defeated, suffering a crushing blow that effectively ended the Grand Principality of Rus'. The defeat led to the collapse of Švitrigaila's short-lived state and ensured the continuation of Lithuanian rule over the Rus' lands. With the victory Sigismund Kęstutaitis solidified his power as Grand Duke, and the Rus' territories were once again integrated into the Grand Duchy of Lithuania. However, these lands would now be firmly under Lithu,anian control, with no significant autonomy for the Rus' people. The failure of the Grand Principality of Rus' als,o marked a significant shift in the governance of the region, as the aspirations of the Rus' nobility for greater independence were suppressed.