After the 2nd Army's annihilation at the Battle of Tannenberg, Rennenkampf's army took over all the defenses along the Deyma, Lava and the Masurian Lakeland. On 7 September, the First Battle of the Masurian Lakes began, as the Germans attacked the left flank of the 1st Army with a powerful detachment, Zhilinsky broke the promise to provide Rennenkampf with reinforcements from other formations. As a result, the 1st Army had to retreat in a hurry. The 2nd Army Corps led by General Vladimir Slyusarenko resisted desperately, so did Rennenkampf himself, who brought all the cavalrymen, reserves, and troops, all transferred from the right to left flank of the 20th Army Corps, in order to avoid encirclement by Hindenburg. By 15 September, he skillfully withdrew his men from encirclement and withdrew behind the Neman, saving all the remaining troops he had. After this failure, despite his best efforts to blame Rennenkampf for the defeat, Zhilinsky was dismissed and was replaced by the General Nikolai Ruzsky.