Complex sentence: At the outbreak of World War I Lieutenant Colonel Hoffmann was the deputy chief of staff of the German Eighth Army, the only German military unit defending East Prussia from a Russian attack.
Split into simpler sentences: At the outbreak of World War I Hoffmann was the deputy chief of staff of the German Eighth Army stationed in East Prussia. During the opening months of the war the Eighth Army was the only German military unit defending East Prussia from a Russian attack.