After serving for eleven years in the
Bavarian Army, Koeth joined the
Prussian Army in January 1900 as a
Premier-Lieutenant. He had attended the
War Academy at Munich for three years (1895–98) but was not awarded the full qualification to join the general staff, despite excellent performance. He then served for nine years as a chief of battery in the 4th
Badische Feldartillerie-Regiment Nummer 66 and in the training regiment
Feldartillerie-Schießschule at
Jüterbog (after 1904). After August 1909, Koeth worked at the
Prussian Ministry of War (
Feldartillerie-Abteilung, A 4). In March 1912, he was promoted to Major. After the start of
World War I, Koeth briefly served as
Abteilungskommandeur (
Battle of the Marne, static battles in the
Champagne). However, in early October 1914 he returned to department A4 in the Ministry. In late February 1915, after working as an acting head of department, he succeeded
Walther Rathenau as head of the
Kriegsrohstoff-Abteilung (KRA, commodity department), without having had any prior experience in this field. His task was to provide war production with the required raw materials. He succeeded in this work through a system of
Planned economy, systematic recycling of used materials and by accessing new sources of commodities. The work of his office (employing around 2,500 people at the end of the war) was highly regarded and brought him the recognition of the military command, industrialists and trade unions. However, in 1916/17 there were sharp differences with general
Wilhelm Groener, head of the
Kriegsamt, in connection with the
Hindenburg Programme and the
Auxiliary Services Act (1916). The cooperation with Groener's successor,
Heinrich Scheuch, went more smoothly. In March 1917, Koeth was promoted to
Oberstleutnant and
Abteilungschef (head of department). ==Demobilisation and political career==