Battle Of Tannenberg
The Battle of Tannenberg was in August 1914 a decisive engagement between the Russian Empire and the German Empire in the first days of World War I, fought by the Russian First and Second Armies and the German Eighth Army between 23 August and 30 August 1914. The battle resulted in the almost complete destruction of the Russian Second Army. A series of follow-up battles destroyed the majority of the First Army as well, and kept the Russians off-balance until the spring of 1915. The battle is notable particularly for a number of rapid movements of complete German corps by train, allowing a single German Army to present a single front to both Russian Armies.
Although the battle took place near Allenstein, Ludendorff's aide Max Hoffmann suggested to name it after Tannenberg in an attempt to erase the defeat in the medieval Battle of Grunwald (Tannenberg) of 1410 in which the Teutonic forces were defeated by the Poles and Lithuanians. As pointed out by Christopher Clark, the actual Tannenberg is some thirty kilometres to the west, and there was no intrinsic reason - other than the historical battle and its emotive resonance in the narrative of German Nationalism - to name for it the 1914 battle.
In the interim, the two Russian armies had drifted so far apart that neither could come to the aid of the other if it were attacked. Hoffman knew this from intercepted radio messages. He also knew of the deep mutual dislike the two Russian commanders had for each other which would further disincline them from supporting each other. Hoffmann was then able to devise a plan for an encirclement victory over Alexander Samsonov's Second Army in the south which Hindenburg quickly put into action upon his arrival leading to the Battle of Tannenberg. After the victory, the Eighth Army turned north and defeated Paul von Rennenkampf's First Army at the First Battle of the Masurian Lakes driving the Russians out of East Prussia for the remainder of the war.
Wikipedia: Battle Of Tannenberg
Wikipedia: World War I