When the war began, the Deutsches Heer did not expect Russian Army forces to be pushing eastern Prussia within 15 days, but they were. With only a few skeleton units to defend it, prospects seemed grim, until Generals Ludendorff and Hindenburg destroyed the First and Second Russian Armies during the bloody Battle of Tannenberg. Scrambling to form a defensive line, the Russians could only halt the German Army's advance until the marshes and forests of eastern Poland and western Belarus.
World War I - The Battle of Tannenberg
World History - World Wars
July 1914 to July 1915