Following the Marne disaster during the first month of the war, Falkenhayn was selected to replace von Moltke as Chief of General Staff. He simultaneously held this position and that of Prussian War Minister for the next five months. Highly intelligent, but indecisive and aloof, his push for unrestricted submarine warfare brought him into conflict with Chancellor von Bethmann-Hollweg.  He also developed the perspective that the Western Front was the most crucial area of fighting, bringing him into bitter conflict with the heroes of the East, von Hindenburg and Ludendorff.  He understood early on, however, that the war was a probably a lost cause, compelling him in 1916 to devise the desperate plan that would become the debacle of Verdun. Because of this, he was replaced by von Hindenburg as Chief of Staff, and thereafter demoted to commander the Ninth Army, a force which overran Romania within nine weeks. He was then transferred to Palestine to command Army Group Yildirim (1917-18) and recapture Mesopotamia, but his failure to halt General Allenby there saw him replaced by the capable Liman von Sanders and sent to the relatively obscure command of the Tenth Army in Lithuania, where he spent the last six months of the war. During the war, Falkenhayn was awarded both the Pour le Merite (16.2.1915) and the Order of the Black Eagle (12.5.1915).

Soon after the war, General of Infantry von Falkenhayn went into retirement and secluded himself at Schloß Lindstedt near Potsdam in order to pen his memoirs. Before dying there on 8 April 1922, he wrote "Supreme Army Command 1914-1916 and its Most Noteworthy Decisions" and "The Ninth Army and its Campaigns Against the Romanians and Russians, 1916-1917". Von Falkenhayn's elder brother Eugen also served during the war as a corps-level commander. He died on 8 April 1922 in Lindsted and is buried at the Bornstedter Friedhof near Schloß Sanssouci in Potsdam.