The Battle of Tannenberg saw the German forces under the command of Paul von Hindenburg inflict one of the most complete defeats in military history on the Russian second army.
As part of a French strategy to remove the Germans from Alsace and Lorraine the Russian First and Second armies were to invade East Prussia. The massive number of troops brought to bear by the Russians dwarfed the German forces in the region, with the Second army alone (230,000 troops) outnumbering the entire German force (150,000 troops).
The Russian plan revolved around trapping the German forces between the First and Second armies, but the German use of their fast rail allowed them to turn the tables on the Russians and fight each army individually.
Von Hindenburg sent one corps via rail unnoticed to the left flank of the Russian Second army before sending the majority of his forces to their right flank. He left a small mobile contingent of troops in the north to keep the Russian First army occupied, using their speed this contingent was able to appear to ab a much bigger force disguising the fact that the remainder of the army had moved against the Second army.
The German force sent to the left flank of the Second army was able to cut off the Russian line of advance and attack the Russian forces from the rear whilst the bulk of the German army engaged from the front and encircled the Russians.
Although severely outnumbered the German Eighth army lost only 10,000-15,000 men (killed and wounded) whereas the Russian Second army lost 78,000 (killed or wounded) and a further 92,000 were taken prisoner. After the defeat the commander of the Russian Second army, Alexander Samanov walked into the nearby forest and shot himself.