
With both messages in hand, Hoffmann promptly hurried after Ludendorff and Hindenburg and handed them the intercepts. Consequently, his message provided detailed plans for his intended route of pursuit of the German forces. Having engaged - unsuccessfully - the heavily entrenched German XX Corps the previous day, 24 August, at the Battle of Orlau-Frankenau, Samsonov had noted what he took to be a general German withdrawal to Tannenberg and beyond. The second intercepted message, from Samsonov, was similarly remarkable. The import of the message was clear: the Germans need not fear intervention from the Russian First Army during their assault upon Samsonov's forces. It further detailed his First Army's imminent marching plans, and these were not towards Samsonov's Second Army. The first, sent by Rennenkampf, revealed the distance between his and Samsonov's armies. Whilst returning from their meeting with Francois, Hoffmann was passed two intelligence intercepts that had been transmitted by Rennenkampf and Samsonov, respectively, in the clear, i.e. Reluctantly, Francois agreed to commence the attack, but complained of a lack of shells. Ludendorff - along with Hoffmann - travelled to see Francois and to repeat the order. Remarkably, Francois rejected what was clearly a direct order, choosing instead to wait until his artillery support was in readiness on 27 August. Ludendorff issued an order to General Francois to initiate the attack on Samsonov's left wing at Usdau on 25 August.

On 22 August the bulk of Samsonov's forces reached the extremities of the German line, fighting (and winning) small actions as it continued to advance into the German trap of encirclement. The remaining VI Corps he directed north towards his original objective, Seeburg-Rastenburg. Assured that his Second Army was en route to pursue and destroy the supposedly retreating Eighth Army (and supported in doing so by overall commander, who was subsequently dismissed for his part in the following debacle), he continued to direct his army of twelve divisions - three corps - in a north-westerly direction towards the Vistula. Samsonov was similarly unaware of Hoffmann's plan or of its execution. Samsonov meanwhile, bedevilled by supply and communication problems, was entirely unaware that Rennenkampf had chosen to pause and lick his wounds at Gumbinnen, instead assuming that his forces were continuing their movement south-west. Finally, a fourth corps was ordered to remain at Vistula to meet Samsonov as his army moved north. Hindenburg's remaining two corps, under Below, were to await orders to move south by foot so as to confront Samsonov's opposite right wing. Meanwhile, General Corps were transported by rail to the far southwest to meet the left wing of Samsonov's Second Army. Hoffmann proposed a ploy whereby cavalry troops would be employed as a screen at Vistula, the intention being to confuse Rennenkampf who, he knew, held a deep personal vendetta with Samsonov (who had complained of Rennenkampf's conduct at the Battle of Mukden in 1905) and so would be disinclined to come to his aid if he had justifiable cause not to. While Hindenburg and Ludendorff received much credit for the subsequent action at Tannenberg, the actual plan of attack was devised in detail by Hoffmann. Upon his arrival in East Prussia on 23 August Hindenburg immediately reversed Prittwitz's decision to withdraw, choosing instead to authorise a plan of action prepared by Colonel, Prittwitz's deputy chief of operations. Upon receipt of this news, the German Army Chief of Staff, recalled Prittwitz and his deputy von Waldersee to Berlin - an effective dismissal - and installed as their replacement the markedly more aggressive combination of brought out of retirement at the age of 66 - and his Chief of Staff (having earlier distinguished himself at ). Prittwitz, shaken by the action at Gumbinnen and fearful of encirclement, ordered a retreat to the River Vistula. Rennenkampf brought about a modification however following a scrappy victory against Eighth Army at the, after which he paused to reconsolidate his forces. The two armies planned to combine in assaulting Eighth Army, Rennenkampf in a frontal attack while Samsonov engulfed Prittwitz from the rear. begun to take his Second Army into the south-western corner of East Prussia whilst into its north-east with the First Army.

Russia's incursion into German territory was two-pronged. Perhaps the most spectacular and complete German victory of the First World War, the encirclement and destruction of the Russian Second Army in late August 1914 virtually ended Russia's invasion of East Prussia before it had really started.
