The Battle of Grunwald or First Battle of Tannenberg or Battle of Žalgiris was fought on 15 July 1410, during the Lithuanian–Polish–Teutonic War. The alliance of the Grand Duchy of Lithuania and the Kingdom of Poland, led respectively by Grand Duke Vytautas (Witold; Vitaŭt) and King Władysław Jagiełło (Jogaila), decisively defeated the German–Prussian Teutonic Knights, led by Grand Master Ulrich von Jungingen. Most of the Teutonic Knights' leadership were killed or taken prisoner. While defeated, the Teutonic Knights withstood the siege on their fortress in Marienburg (Malbork) and suffered only minimal territorial losses at the Peace of Thorn (1411) (Toruń). Territorial disputes continued until the Peace of Melno was concluded in 1422. However, the Knights never recovered their former power and the financial burden of war reparations caused internal conflicts and an economic downturn in their lands. The battle shifted the balance of power in Eastern Europe and marked the rise of the Polish–Lithuanian union as the dominant political and military force in the region.

The battle was one of the largest battles in Medieval Europe and is regarded as the most important victory in the history of Lithuania, Poland and Belarus. It was surrounded by romantic legends and source of national pride, becoming a larger symbol of struggle against invaders. During the 20th century, the battle was used in Nazi and Soviet propaganda campaigns. Only in recent decades have historians made progress towards a dispassionate, scholarly assessment of the battle reconciling the previous narratives, which differed widely by nation.