The Battle of Tannenberg
Contrary to German expectations, the Russians had managed to get a good portion of their army into the field quite quickly following the German declaration of war. Already by the second week of the war, while the Germans were still caught up in Belgium, the Russians had dispatched a force of 500,000 men into eastern Prussia. The approach of this Russian force so frightened German Supreme Command that they would pull some of their forces off of the western front and send them to Prussia, when these same forces may have been decisive had they remained in the west and participated in the Battle of the Marne.
The Russian forces threatening Prussia were led by Paul von Rennenkampf and Alexander Samsonov, two otherwise competent commanders who thoroughly despised one another. The Germans fielded their own duo to oppose the Russians: Erich von Ludendorf, who had recently proved himself in Belgium, and Paul von Hindenburg, a veteran of the German wars of unification who had been called out of retirement to face the Russians.
When the Battle of Tannenberg began on August 26, the Russians outnumbered the Germans two to one, but Ludendorf and Hindenburg had some advantages nonetheless. This was Prussia, after all, the homeland of the German officer corps. They knew the terrain, they had an impeccable railroad network, and they had airplanes and zeppelins for reconnaissance while the Russians were advancing blind. The territory through which the Russians advanced was also broken up by the Masurian Lakes, forcing Rennenkampf and Samsonov to divide their forces.
Hindenburg and Ludendorf, having intercepted Russian radio transmissions, knew that Samsonov was advancing in the south on his own and that he was not interested in linking back up with Rennenkampf. The Germans elected to lay a trap for Samsonov, moving the entirety of their force south and allowing Samsonov to continue to advance until he became encircled. The Germans then sprung their trap, destroying Samsonov’s army before turning to face Rennenkampf, eventually driving him off. As a result of the battle Prussia was saved and the Russians lost 300,000 men. Samsonov, unable to handle the shame of defeat, committed suicide shortly thereafter.