Battle of Gumbinnen The Battle of Gumbinnen , initiated by forces of the German Empire on August 20 , 1914 , was the first major German offensive on the Eastern Front during the First World War . Because of the hastiness of the German attack , however , the Russian army emerged victorious . Background . At the outbreak of the war , Maximilian von Prittwitz ' s orders were very strict and clear : his German Eighth Army was to remain in its positions in East Prussia , without attempting any offensive action , as all German efforts were to be concentrated on the Western Front against France , according to the Schlieffen Plan . In addition , should the Russians increase their pressure , he was authorized to fall back as far as the Vistula River , abandoning eastern Prussia . The Eighth Army comprised four corps : I Corps ( Hermann von François ) , XVII Corps ( August von Mackensen ) , I Reserve Corps ( Otto von Below ) , and XX Corps ( Friedrich von Scholtz ) , plus 1st Cavalry Division , facing the Russian First Army ( Paul von Rennenkampf ) and Second Army ( Alexander Samsonov ) . The Russians enjoyed considerable numerical superiority , but were hampered by significant deficiencies in their services of supply and field communications . François was convinced that German training and equipment made up for their lack of numbers , and was pressing for offensive action . On the 17th he launched , on his own initiative and against orders , an attack against the Russian First Army at the Battle of Stallupönen . By the time he withdrew to Gumbinnen after this battle , his corps had inflicted 5 , 000 casualties and managed to capture about 3 , 000 Russian prisoners . German attack and retreat . With this success , François persuaded Prittwitz to launch an offensive against the Russian First Army while the Second Army was still far to the south . François argued that his troops , many of whom were native East Prussians , would be demoralized by retreating and leaving their homeland to the Russians , and that the Russians were not as strong as they appeared to be . Prittwitz was convinced , and decided to engage Rennenkampf at the earliest possibility , pitting 150 , 000 Germans against 200 , 000 Russians . This decision went against the orders of Helmuth von Moltke , the German Chief of Staff , which specifically ruled out any offensive on the Eastern Front until France was defeated in the West . On August 19 , Russian cavalry came into contact with a German infantry regiment outside Gumbinnen . Instead of withdrawing , the Russians dismounted and brought up their artillery to continue the fight , driving the Germans back . However , they suffered 400 casualties and after expending most of their ammunition were forced to retreat themselves . This was the signal François had been awaiting , and he convinced Prittwitz to launch a counterattack the next day . With Prittwitz ' s approval , François started moving I Corps forward that night , reinforced by the 1st Cavalry Division . At 04 : 00 on August 20 , I Corps attacked the Russian 28th Division , which put up a spirited artillery defense . However , the Russians were always lacking in supplies , and they soon expended their artillery ammunition . This left them at the mercy of the German artillery , and they were forced to retreat 8 km in the early afternoon . The lines were stabilized when the Russian 29th Division arrived , and the battle turned into a stalemate . To the south , Mackensen ' s XVII Corps and Below ' s I Reserve Corps were still moving up and were not ready for combat . Hearing of von François ' s actions further north , von Mackensen attacked Rennenkampf ' s III Corps at 08 : 00 , but von Below was not able to join in until noon . The Russians in this area were well aware of German intentions due to von François ' s attack , and had spent the time preparing for the assault by moving up their heavy artillery . At first the German advance went well , but faltered once they came under Russian artillery fire , and the Russians were able to turn their flanks and force them to retreat in disorder to the Insterburg - Angerburg lines , leaving 6 , 000 prisoners in Russian hands . " The uncharacteristic sight of defeated German soldiers streaming mob - like to the rear really unnerved Prittwitz " , who feared that his army could be trapped between Rennenkampf and Samsonov , although the former did not seem eager to pursue the retreating German troops . Prittwitz panicked and , with a decision out of proportion to the severity of the situation , ordered a general retreat to the Vistula , leaving East Prussia to the Russians . Helmuth von Moltke ' s reaction . Prittwitz ' s panic affected Moltke , who feared that Berlin itself could now be threatened by the advancing Russians . The Chief of Staff reacted by removing Prittwitz and his deputy , Waldersee , replacing them with Paul von Hindenburg and Erich Ludendorff . He also transferred several divisions from the Western Front , which has been generally considered to have been an incorrect decision , as it weakened ( some scholars say fatally ) the German " marching wing " that was intended to rapidly move across Belgium to outflank and destroy the French army . One seemingly minor outcome of the battle would have lasting effects . After the battle , a note was found on a dead Russian officer that outlined the greater part of the Russian plans for the campaign . As Hindenburg recalled : Armed with this intelligence , Hindenburg and Ludendorff halted the German retreat and decided to take the initiative . This would result in the Battle of Tannenberg , one of Germany ' s greatest victories .