Following the Union of Krewo in 1385, tensions between Jogaila and Vytautas escalated, leading to the Lithuanian Civil War (1389–1392)—a second round in their ongoing power struggle. Although Jogaila had become King of Poland in 1386, his attempts to secure control over Lithuania by appointing his brother Skirgaila as regent were unpopular among Lithuanian nobles. Many resented the growing Polish influence over Lithuanian affairs and favored Vytautas, who sought to reclaim his ancestral lands and maintain Lithuania’s autonomy within the union.
When Vytautas’s first attempt to seize Vilnius failed in 1389, he turned once again to the Teutonic Knights—just as both he and Jogaila had done in the earlier civil war (1381–1384). In 1390, Vytautas and the Knights besieged Vilnius but failed to capture it. The campaign demonstrated deep dissatisfaction among Lithuania’s elites with Jogaila’s leadership, though no side achieved a decisive victory.
By 1392, with neither faction gaining the upper hand, Jogaila offered a compromise. In the Ostrów Agreement, Vytautas was named Grand Duke of Lithuania under the condition that he would recognize Jogaila as Supreme Duke. Vytautas accepted, abandoned his alliance with the Knights, and turned on their strongholds, marking the end of the civil war.
Though technically a vassal of Jogaila, Vytautas wielded considerable autonomy and became the de facto ruler of Lithuania. His leadership helped stabilize the Grand Duchy and restore unity, much like the earlier power-sharing arrangement between their fathers, Algirdas and Kęstutis. Vytautas’s reign (1392–1430) allowed Lithuania to refocus on external threats, particularly the Teutonic Knights, who were enraged by Vytautas’s betrayal. Though Vytautas temporarily ceded Samogitia to the Knights in the Treaty of Salynas (1398) to buy time, tensions persisted. The civil war laid the foundation for the renewed cooperation between Jogaila and Vytautas.