Already aged 65 at the outbreak of war in 1914, Mackensen's XVII Army Corps became part of the German Eighth Army in East Prussia, under General Maximilian von Prittwitz and, 21 days later, under General Paul von Hindenburg. Mackensen had his corps moving out on a 25 km march to the Rominte River within fifty minutes of receiving his orders on the afternoon of 19 August 1914, after the Imperial Russian Army invaded East Prussia. He led the XVII Corps in the battles of Gumbinnen, Tannenberg and the First Battle of the Masurian Lakes, which drove the invading Russians out from most of East Prussia.