When was Russia defeated at tannenberg?

Russia was decisively defeated at the Battle of Tannenberg from August 26 to August 30, 1914, during World War I. The battle, fought between the German Empire and the Russian Empire, resulted in a significant victory for the Germans, leading to the encirclement and destruction of the Russian Second Army. This defeat had a profound impact on Russian military morale and strategy in the early stages of the war.