 Complete the following phrase:  When the Russian invasion threatened his rear, Prittwitz suggested a retreat to the west of the Vistula. This meant abandoning East Prussia, which the German General Staff found unacceptable. Additionally, commander of the I Corps Hermann von François complained to the General Staff that his superior was panicking; the General Staff concurred in this assessment. Prittwitz was promptly replaced as Eighth Army commander by Paul von Hindenburg on 23 August 1914. Hindenburg, and his chief of staff Erich Ludendorff, then destroyed the two invading Russian armies at the Battles of Tannenberg and the 