The Battle of Tannenberg of 1914 was a decisive conflict between Russia and Germany in the first days of World War I. The Russian 1st and 2nd armies and the German 8th Army fought from August 17 to September 2, 1914.
The Russian armies crossed into East Prussia with Königsberg as their goal. At first, the battle appeared to go well for the Russians, with the German's first counterattack repulsed on August 20. The German theatre commander, General Maximilian von Prittwitz, was sacked when he attempted to completely abandon East Prussia to the Russians.
While replacements were en route, the 8th Army's Chief of Operations, Max Hoffmann, redeployed the army. Hoffmann's plan left a screening force to delay the Russian 1st Army (under General Pavel Rennenkampf) which was approaching from the east, and set a trap for the Russian 2nd Army (under General Alexander Samsonov) which was moving up from the south.
The German field commander, General Hermann von Francois, allowed the 2nd Army to advance, and then cut them off from their already tenuous supply route. This forced massive surrenders, and saw the almost complete destruction of the 2nd Army near Frogenau. However, General Erich von Ludendorff, the chief of staff for new theatre commander Field-Marshal Paul von Hindenburg dated the official dispatch reporting the victory from the nearby village of Tannenberg (Stębark), and the battle is thus known to history.
The German victory compelled Rennenkampf to withdraw his army from East Prussia, and thus cleared German territory of invaders. The Russians remained on the defensive along the German front for the rest of the war.
See also: Battle of Tannenberg/Grunwald (1410)