Allied with France and Britain, Grand Duke Nicholas, the Russian commander, agreed to help relieve the French, under attack from Germany, with an offensive in East Prussia. This required mobility and nimbleness; unfortunately the Russians had neither.
Two Russian armies invaded German East Prussia in August 1914. Rennenkampf's First Army was to converge with the Samsonov's Second Army to give a two-to-one numerical superiority over the German 8th Army, which they would attack from the east and south respectively, some 80km (50 miles) apart.
The plan began well at Gumbinnen on 20 August, when Rennenkampf's First Army defeated eight divisions of the German 8th Army on its eastern front. By this time Samsonov's forces had crossed the southern frontier of East Prussia to threaten the German rear, defended by only three divisions.
Faced with imminent attack, Prittwitz, commander of the 8th Army, approved Lieutenant Colonel Hoffman's idea to attack Samsonov's left flank, aided by another three divisions moved by rail from the Gumbinnen front. However, on 23 August Prittwitz was replaced by General von Hindenburg whose chief of staff, Ludendorff, immediately confirmed Hoffmann's plan to strike at Samsonov's left flank.
The Germans then got lucky when they intercepted an uncoded Russian message indicating that Rennenkampf was in no hurry to advance. Developing Hoffman's original plan, Ludendorff concentrated six divisions against Samsonov's left flank and took a calculated risk to withdraw the rest of the German troops from Gumbinnen and move them to face Samsonov's right flank, leaving only a cavalry screen against Rennenkampf. This move was helped by the lack of communication between the two Russian commanders, who disliked each other.
Samsonov's forces were spread out along a 60 mile front and advancing gradually against the Germans when, on 26 August, Ludendorff ordered an attack on Samsonov's left wing near Usdau. There, German artillery forced a Russian retreat, whereupon they were pursued toward Neidenburg, in the rear of the Russian centre.
A Russian counter-attack from Soldau enabled two Russian army corps to escape south east before the German pursuit continued. By nightfall on 29 August the Russian centre, amounting to three army corps, was surrounded by Germans and stuck in a forest with no means of escape. The Russians disintegrated and were taken prisoner by the thousands. Faced with total defeat, Samsonov shot himself. By the end of the month, the Germans had taken 92,000 prisoners and annihilated half of the Russian 2nd Army. Rennenkampf's army had not moved at all during this battle, vindicating Ludendorff's calculated risk.
After being reinforced, the Germans turned on Rennenkampf's slowly advancing Army, attacking it in the first half of September and driving it from East Prussia. It was a crushing defeat for the Russians. In total, they lost around 250,000 men - an entire army - as well as vast amounts of military equipment. The wafer-thin silver lining was that the Russian action had diverted the Germans from their attack on France and allowed the French to counter-attack at the Marne.