Russian invasion of East Prussia ( 1914 ) The Russian invasion of East Prussia occurred during the First World War , lasting from August to September 1914 . As well as being the natural course for the Russians to take upon the declaration of war with Germany , it was also an attempt to focus German military eyes on the Eastern Front , as opposed to the Western Front , where France was increasingly under the strain of her own German invasion . Plans . The Germans initially planned to have only the 8th army to act as a bulkwark against any Russian incursion . However , the 9th Army was stationed in central Germany to reinforce the 8th if needed . It was expected that the Russians would be slow to mobilise , leaving Germany to beat France in a few weeks thereby allowing the victorious , battle - hardened German troops to transfer along Germany ' s superior transport network to fight the Russians on the Eastern Front . This was the basis of the Schlieffen Plan . Battle . However , quite unexpectedly quickly , Russia was able to mobilise an invasion into East Prussia . Any invasion of Prussia was an important blow to German morale as well as her general strategic situation , due to Prussia ( including East Prussia ) being the historical heart of the German " Reich " ( Empire ) . The German deployment on the outbreak of the war left only the 10 divisions of the German Eighth Army under General Maximilian von Prittwitz in East Prussia whereas the Russians had been able to mobilise the First Army , under General Paul von Rennenkampf and the Second Army , under General Alexander Samsonov . They entered East Prussia on 7 – 9 August . The Battle of Stallupönen , fought between Russian and German armies on August 17 , 1914 , was the opening battle of World War I on the Eastern Front . It was a minor German success , but did little to upset the Russian timetables . The Battle of Gumbinnen , started by the Germans on August 20 , 1914 was the first major offensive on the Eastern Front during the First World War . Due to the hastiness of the German attack the Russian army emerged victorious . The Germans were forced to retreat , perhaps with the intention of performing holding actions in Mazuria , or even retreating to the River Vistula which would have meant abandoning the salient of East Prussia . This would have fitted in with the plans made before the start of the First World War ; that these were the positions the Germans would retreat to if the Russians put up a much stronger fight than they had anticipated . Regardless of whatever preparations had been made , however , it still remained that the Germans could not let the Prussian capital , Königsberg fall into Russian hands . The moral , symbolic and military value ( since it was a major military hub ) of the city meant to lose it was to invite disaster on the home front , in addition to the strategic ramifications . Also , it was very likely that the Russians would have used the upper hand thus gained to use their superior forces to overwhelm the static German defences . In short , the Germans had to fight back immediately and force the Russians from East Prussia . Helmuth von Moltke the Younger , Chief of the German General Staff from 1906 to 1914 replaced Prittwitz with Paul von Hindenburg ( brought out of retirement ) on the 22nd August . Hindenburg , along with his Chief of Staff , the formidable Ludendorff would approach the crisis in East Prussia very differently from Prittwitz , who panicked when the Russian onslaught entered East Prussia . In contrast to Prittwitz , Hindenburg and Ludendorff decided to take the offensive and encircle one of the opposing armies . Following the plans of Colonel Max Hoffmann , Prittwitz ' s deputy chief of operations , they chose to send eight divisions against Samsonov in the Battle of Tannenberg resulting in over 90 , 000 captured and 70 , 000 killed or wounded . The Second Army was destroyed and Samsonov shot himself . In the Battle of the Masurian Lakes , the Germans forced the First Army to retreat out of East Prussia . The invasion was a ghastly failure for the Russians , a setback which was followed by considerable German advances in the following year , including the capture of the Polish city of Warsaw . However , the crisis caused in the German High Command by the unexpected Russian advance forced the sending of three corps and a cavalry division from the Western Front as part of the new 9th Army in order to support the attack on the Russians . These additional forces didn ' t arrive in time for the twin battles , as Ludendorff predicted and , had they entered France as originally planned could have been tremendously helpful to the precarious situation in the West . In the Head of French Intelligence Colonel Dupont ' s words , " their debacle was one of the elements of our victory . "