Tannenberg, Battle of, was a clash between German and Russian troops during World War I (1914-1918). The battle took place from Aug. 26 to Aug. 31, 1914, and ended in a decisive German victory. At that time, Tannenberg—now Stębark, Poland —was a town in East Prussia , a province of the German Empire. The German victory at Tannenberg stopped an early Russian advance into East Prussia.
Background.
World War I began in early August 1914. Germany went to war against Russia , France , and the United Kingdom . Germany hoped to defeat France quickly while Russia mobilized (called up troops). However, Russia mobilized quickly and invaded East Prussia. 
Two Russian armies, each totaling some 200,000 troops, entered East Prussia to the north and south of the Masurian Lake District , an area of forests, lakes, and hills. The Russian force was strong, but it faced a number of problems. The two armies were separated by more than 50 miles (90 kilometers) and had little communication. They were low on supplies and moved largely on foot—often just a few miles a day. This was partly due to the difference in railway track gauges (distance between the rails) in Russia and East Prussia. This difference—and the German removal of local train cars and locomotives—prevented the Russians from using German railways. Lastly, Russian reconnaissance (information gathering) relied on cavalry . It often failed to keep track of the enemy.
With most German troops fighting in Belgium and France, a single army of some 200,000 soldiers faced the Russians in East Prussia. German communications and supply were excellent, however. Army units moved quickly over an extensive rail network. A number of airplanes—as well as intercepted Russian radio messages—helped the cavalry with reconnaissance. German troops, defending their own territory, knew the terrain.
On August 17, the Germans won a minor victory over the invading Russians at Stallupönen (now Nesterov, Russia). It was the first battle on the Eastern Front, a battlefront that would eventually stretch from Estonia to Romania . Russian forces then won a larger battle at Gumbinnen (now Gusez, Russia), and threatened Königsberg (now Kaliningrad, Russia), the East Prussian capital.
Fearing the loss of East Prussia, the German army chief of staff, Helmuth von Moltke , sent two new generals— Paul von Hindenburg and Erich Ludendorff —to take command. Moltke also ordered reinforcements rushed from Belgium and France. Hindenburg and Ludendorff called off a planned retreat. Realizing the Russian armies were too far apart to aid one another, the Germans decided to attack them one at a time.
The battle.
On August 26, German forces, with strong artillery support, attacked the southern Russian army. The Russian line, surprised and strung out for 60 miles (100 kilometers), collapsed. German troops quickly advanced on each flank (side), encircling a large pocket of Russian troops. The Russians began to withdraw on August 28, but much of their force was surrounded. In the following days, trapped Russian units surrendered or tried to break through the German lines. Of the many troops caught in the pocket, just 10,000 escaped.
Aftermath.
More than 90,000 Russians became prisoners of war at the Battle of Tannenberg. Some 50,000 were killed or wounded. German casualties (people killed, wounded, missing, or captured) totaled less than 20,000.
In early September 1914, the Germans—now strongly reinforced—defeated the northern Russian army at the First Battle of the Masurian Lakes. The Russians withdrew from East Prussia, but soon invaded again. The Russians left East Prussia for good, however, following defeat in the Second Battle of the Masurian Lakes in February 1915. Generals Hindenburg and Ludendorff, boosted by their success against Russia, went on to lead the entire German war effort.
