Erich Ludendorff
Erich Ludendorf was a German general during World War I. Along with Hindenburg, he won a spectacular victory over the Russians early in the war at the Battle of Tannenberg.
Following the Battle of Verdun, Hindenburg and Ludendorff were put in charge of the whole German army. Tactically, they did well, leading the Germans to several victories.
Strategically, Ludendorff was a huge blunderer. He was in part responsible for the decision to launch unrestricted submarine warfare, bringing the United States into the war in late 1917. In the spring of 1918, he frittered away Germany's strength by occupying huge stretches of Russia, and mounting massive offensives in the west. Although the Germans gained some ground, their losses were irreplaceable, and by the fall of 1918 the Allies steamrolled to victory.
Later Ludendorff associated himself with Hitler and tried to seize power in the failed Beer Hall Putsch.