At the beginning of World War, the XIII Army Corps commanded by von Fabeck was part of Germany's 5th Army which was commanded by Crown Prince Wilhelm. It participated in the mobile battles known as the Race to the Sea and the First Battle of Ypres. In March 1915 von Fabeck briefly commanded the newly formed 11th Army, which was quickly transferred from the Western to the Eastern fronts with whom he fought in Lithuania. In April 1915 he replaced the injured Alexander von Kluck as commander of the 1st Army. In September 1915 von Fabeck got command of the 12th Army, with whom he transferred to the Eastern Front. He was also attached à la suite to Infanterie-Regiment Nr. 129 on 27 January 1916. Before he fell ill in October 1916 von Fabeck was the commander of 8th Army for a few weeks.