Русский корпус
Von François was ready to attack the Russian left decisively on 27 August, hitting I Russian Corps. His artillery barrage was overwhelming, and soon he had taken the key town of Usdau. In the center the Russians continued to strongly attack the German XX Corps and to move northwest from Allenstein. The German XVII Corps and I Reserve Corps pushed the Russian right wing they had bloodied the day before further back. Gen. Basil Gourko, commanding the Russian First Army Cavalry Division (and from 1916 to 1917 chief of the general staff), was told later that Samsonov did not know what was happening on his flanks because he was observing the action from a rise in the ground a distance from his wireless set and reports were not relayed to him.