General Francois, a commander on the winning side 
General Francois, a commander on the winning side of this battle, was ultimately able to take Soldau. It was made possible due to the recall of von Waldersee and Prittwitz. One side was able to use data obtained from unencrypted wireless signals to learn that the enemy's Second Army would not be joined by General Rennenkampf. As a consequence of this battle, A. V. Samsonov shot himself after surrendering some 95,000 troops, and Ludendorff and Hindenburg built prestige. For 10 points, identify this early defeat for the Russians in World War I, also the site of a medieval defeat for the Teutonic Knights.
General Francois, a commander on the winning side of this battle, was ultimately able to take Soldau.
Second Battle of Tannenberg [or Battle of Stebark]