General-Adjutant in 1912. A further promotion to commander-in-chief of all troops within the Vilna Military District followed in mid-January 1913. World War I In the beginning of the First World War, Rennenkampf was appointed commander of the 1st Army of the Northwestern Front, under commander-in-chief General Yakov Zhilinsky; his rival Samsonov was the commander of the 2nd Army invading East Prussia from the south. On 7 August 1914, Rennenkampf and his troops entered East Prussia from the east. While retreating upon orders from the defending 8th Army commander, General Maximilian von Prittwitz, a gap formed between the Russian armies; exploiting this, the Imperial German 1st Division, commanded by General Hermann von François, counterattacked at the Battle of Stallupönen, initiating the Eastern Front of the war. Although it was a victory for the Germans, Russian artillery bombardment halted the German offensive, causing François to withdrew to the town of Gumbinnen. Rennenkampf continued his advance, defeating the 8th Army under the Prittwitz's command at the Battle of Gumbinnen. But due to a later incorrect assessment made by Zhilinsky, the victory at Gumbinnen did not develop. After the battle, Rennenkampf was ordered by Zhilinsky to launch an offensive against the heart of East Prussia, Königsberg, but his army did not link up with Samsonov's 2nd Army due to a mistake made by Zhilinsky. As a result, the German 8th Army under the new commander, General (later field marshal) Paul von Hindenburg, charged through the gap, then encircled and nearly wiped out the 2nd Army near Allenstein (the Battle of Tannenberg). When the desperate Samsonov sent his appeals for help, instead of immediately replying and heading south to aid Samsonov, Rennenkampf almost completely ignored the appeal. When he finally began to head south, he and his men started too late and proceeded more slowly than they should have, contributing to the defeat of the 2nd Army. After the battle, Samsonov, fearing being held responsible for the defeat, committed suicide. 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 his promise to provide Rennenkampf with reinforcements from other formations; as a result, the 1st Army had to retreat hurriedly. The 2nd