On 2 November 1914 Mackensen took over command of the Ninth Army from Hindenburg, who became Supreme Commander East (Oberbefehlshaber Ost).
2. novembril 1914 võttis Mackensen kindral von Hindenburgilt üle 9. armee juhtimise, kes omakorda nimetati Idaalade peavalitsejaks (Oberbefehlshaber Ost).