When the two Russian armies first moved into East Prussia to form a pincer attack, Hindenburg completely defeated the Russian Second Army at the Battle of Tannenberg in August 1914. There the German victory came when German general Hermann von François disobeyed orders and attacked the Russian flank. Hindenburg then turned his army north to attack the Russian First Army at the Masurian Lakes in September. François played a major part in this second victory when his First Corps forced back the Russian flank. Under their general, Paul von Rennenkampf, the Russians dug in. He made use of the lakes, rivers and other natural obstacles. Hindenburg used frontal attacks to hold the Russians while his armies tried to surround the Russian trenches. Using a counterattack to hide his movements, Rennenkampf moved his army back to prevent them being surrounded. Over a period of several days the Russian army lost ground. The Russians fought hard but were driven back to the Russian border. Rennenkampf kept his troops in good order as they retreated.