Original: The Battle of Tannenberg was an engagement between the Russian Empire and the German Empire in the first days of World War I/O. It was fought by the Russian First and Second Armies against the German Eighth Army between 23 August and 30 August 1914.
Simplified: The Battle of Tannenberg was a decisive engagement between the Russian Empire and the German Empire in the first days of World War I, fought by the Russian First and Second Armies and the German Eighth Army between 17 August and 2 September 1914.