In 1435 Polish troops under the command of Kobylański were sent as aid to Sigismund Kęstutaitis (Zygmunt Kiejstutowicz), who was involved in a civil war in Lithuania against his cousin Švitrigaila (Świdrygiełło). Kobylański commanded the combined Polish-Lithuanian force at the Battle of Wilkomierz (present day Ukmergė) where he won a victory over Švitrigaila and the Teutonic Knights who supported him. The outcome of the battle had a significant impact on the civil war in Lithuania, as well as the war between the Crown of Poland and the Monastic State of the Teutonic Knights. Most of the top commanders of the Livonian branch of the order perished in the battle.