The Battle of Tannenberg, fought from August 23 to 30, 1914, during the early stages of World War I, was a significant German victory against Russia. This battle resulted in the catastrophic defeat of the Russian Second Army and the subsequent suicide of its commander, General Alexander Samsonov. Additionally, the encounter led to severe losses for the Russian First Army in the subsequent First Masurian Lakes battles, effectively destabilizing the Russian military efforts in the region until the spring of 1915.

The battle showcased the effectiveness of the German Eighth Army's strategic use of railroads to facilitate rapid troop movements, which was critical in their ability to engage and defeat the Russian armies sequentially. Initially, the Germans managed to delay the Russian First Army, then concentrated their forces to encircle and annihilate the Second Army, and finally returned their focus to the First Army. A significant flaw in the Russian strategy was their failure to encrypt radio communications, instead broadcasting operational plans openly, which the Germans exploited to ensure they faced no surprises in their movements.

The victory at Tannenberg was instrumental in boosting the reputations of Field Marshal Paul von Hindenburg and his staff officer Erich Ludendorff, both of whom became prominent military leaders in Germany. Despite the battle occurring near Allenstein (now Olsztyn), it was named after the historic Tannenberg, site of a medieval battle where the Teutonic Knights were defeated, symbolically linking this modern victory with a historical revenge, thus enhancing its psychological impact and prestige.