Following World War I's outbreak in July 1914, he was recalled to military service and quickly achieved fame on the Eastern Front as the victor of Tannenberg. Subsequently, he oversaw a crushing series of victories against the Russians that made him a national hero and the center of a massive personality cult. By 1916, Hindenburg's popularity had risen to the point that he replaced General Erich von Falkenhayn as Chief of the Great General Staff. Thereafter, he and his deputy, General Erich Ludendorff, exploited Emperor Wilhelm II's broad delegation of power to the German Army to establish a "de facto" military dictatorship that dominated national policy for the rest of the war. Under their leadership, Germany secured Russia's defeat in the east and achieved advances on the Western Front deeper than any seen since the conflict's outbreak. However, by the end of 1918, all improvements in Germany's fortunes were reversed after the German Army was decisively defeated in the Second Battle of the Marne and the Allies' Hundred Days Offensive. Upon his country's capitulation to the Allies in the November 1918 armistice, Hindenburg stepped down as Germany's commander-in-chief before retiring once again from military service in 1919.