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, XVII Corps, I Reserve Corps, and XX Corps, plus 1st Cavalry Division, facing the Russian First Army and Second Army. 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. A radio message that did not use codes supported this assessment. 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 19 AugustRussian 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 20 August, 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 three corps and a cavalry division from the Western Front, which has been generally considered to have been an incorrect decision, as it weakened the German "marching wing" that was intended to rapidly move across Belgium to outflank and destroy the French army.
