The Battle of Tannenberg was an engagement between Russia and Germany in the first month of World War I. Fought between 26 August and 30 August 1914,[8] the battle resulted in the almost complete destruction of the Russian Second Army, and the suicide of its commanding general, Alexander Samsonov. A series of follow-up battles (1st Masurian Lakes) destroyed most of the First Army as well, and kept the Russians off-balance until the spring of 1915. The battle is particularly notable for fast rail movements by the Germans, enabling them to concentrate against two Russian armies in turn, and also for the failure of the Russians to encode their radio messages. It brought high prestige to the rising staff-officer Ludendorff and to Hindenburg who had been brought out of retirement to supervise him.

Although the battle actually took place near Allenstein (Olsztyn), Ludendorff named it after Tannenberg, 30 km to the west, to avenge the defeat of the Teutonic Knights at the earlier battle of that name in 1410.

Strenght

• Russian Second Army (230,000) 
• German Eighth Army (150,000)

Russia 78,000 killed or wounded; 92,000 POW; 350 guns captured

• total 170,000

Germany Official German data 21-30/08/14: 1,726 KIA; 7,461 WIA; 4,686 MIA;

• total 13,873