Jogaila’s appointment of Skirgaila as regent in Lithuania triggered opposition from Vytautas, culminating in the Lithuanian Civil War (1389–1392). The Ostrów Agreement of 1392 resolved the conflict, making Vytautas the Grand Duke of Lithuania under Jogaila’s nominal authority. Vytautas took full control of the Grand Duchy, centralizing power by reclaiming provinces from Ruthenian dukes and consolidating his authority over Lithuanian territories. The Catholicized Lithuanian nobility also became more influential in state politics during his reign.
However, Vytautas rejected the idea of Lithuania’s subordination to Poland, striving to maintain the duchy’s independence while relying on Polish support when necessary. This autonomy allowed Vytautas to expand Lithuania's eastern borders, capturing Smolensk in 1395 and launching a campaign against the Golden Horde. Although Vytautas suffered a crushing defeat at the Battle of the Vorskla River (1399), the duchy remained intact, and he realized that a permanent alliance with Poland was essential for security.
Renewed Conflict with the Teutonic Order
While Vytautas focused on consolidating his rule, the Teutonic Knights intensified their campaigns to gain control over Samogitia. Lithuania and Poland’s union undermined the Order’s mission to convert pagan lands, and they refused to recognize Jogaila's Christianization of Lithuania. The Knights aimed to unite their Prussian and Livonian territories by subjugating Samogitia, a region they briefly secured through the Treaty of Salynas in 1398. Vytautas, however, sought to reclaim Samogitia, prompting further conflict with the Order.
Tensions boiled over in 1409, when Samogitians, supported by Vytautas, revolted against the Knights. Poland backed Lithuania’s cause, and the Teutonic Order responded by invading Greater Poland and Kuyavia, triggering the Polish-Lithuanian-Teutonic War (1409–1411). Both sides prepared for a decisive confrontation, knowing the conflict would determine the regional balance of power.
Battle of Grunwald
On July 15, 1410, the combined Polish-Lithuanian forces, led by Jogaila (Władysław II) and Vytautas, confronted the Teutonic Order at the Battle of Grunwald (Žalgiris/Tannenberg). The battle, one of the largest in medieval European history, ended in a decisive victory for Poland and Lithuania. Most of the Teutonic leadership, including Grand Master Ulrich von Jungingen, was killed or captured. However, despite their battlefield success, the allies failed to capture the Teutonic capital of Marienburg (Malbork Castle) in the subsequent siege.
The Peace of Thorn (1411) formally ended the war, with Samogitia returned to Lithuanian control—though only until the deaths of Jogaila and Vytautas. The Teutonic Order, though not completely dismantled, never recovered its former power. Weakened financially and politically, the Order entered a period of decline, ending its centuries-long dominance over the Baltic region. The victory at Grunwald shifted the balance of power in Central and Eastern Europe, marking the rise of the Polish-Lithuanian union as a dominant force.
Vytautas and Jogaila’s cooperation, symbolized by the triumph at Grunwald, secured Lithuania's independence and territorial integrity, ensuring that the Teutonic Knights would no longer threaten the duchy’s survival. Vytautas continued his campaigns to expand Lithuania’s eastern influence, but Grunwald solidified the union’s dominance in the region and ensured Lithuania’s future as a significant political and military power.