The Battle of Grunwald or First Battle of Tannenberg or Battle of Zalgiris (German: Schlacht bei Tannenberg) is fought on July 15, 1410, during the Polish–Lithuanian–Teutonic War.

The alliance of the Kingdom of Poland and the Grand Duchy of Lithuania, led respectively by King Władysław Jagiełło and Grand Duke Vytautas (Witold), decisively defeats the German-Prussian Teutonic Knights, led by Grand Master Ulrich von Jungingen.

Most of the Teutonic Knights' leadership are killed or taken prisoner.

While defeated, the Teutonic Knights withstand the siege on their fortress in Marienburg (Malbork) and suffer only minimal territorial losses at the Peace of Thorn (1411) (Toruń).

Territorial disputes continue until the Peace of Melno is concluded in 1422.

However, the Knights never recover their former power and the financial burden of war reparations causes internal conflicts and an economic downturn in their lands.

The battle shifts the balance of power in Eastern Europe and marks the rise of the Polish–Lithuanian union as the dominant political and military force in the region.

The battle is one of the largest battles in Medieval Europe and is regarded as the most important victory in the history of Poland, Belarus and Lithuania.

During the 20th century, the battle is 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 differ widely by nation.