Complex sentence: From August 1916 his appointment as Generalquartiermeister made him joint head (with von Hindenburg) of Germany's effort in World War 1 until his resignation in October 1918.
Split into simpler sentences: From August 1916 his appointment as Generalquartiermeister made him joint head (with von Hindenburg) of Germany's war effort. From this point on he ran Germany's war effort in World War I until his resignation in October 1918.