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. Due to the hastiness of the German attack, however, the Russian army emerged victorious.
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, plus one cavalry division, facing the Russian First Army (Paul von Rennenkampf) and Second Army (Alexander Samsonov). The Russians enjoyed considerable numeric superiority, but were hampered by signifcant 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.