The Russians crossed the German border on August 17th 1914 earlier than the German had expected. The Schliefen Plan which the Germans were implementing had called for the bulk of the German army to attack France and then after defeating the French. As a result there were only Eighth Army. The Russian First Army led the attack and they outnumbered the Germans
The first encounter took place on August 20th with the Germans winning a quick tactical victory. However, their troops were overextended and pulled back. Between August 26- 30 the decisive battle took place. The Germans were able to maneuver their troops quickly while the Russians were unable to move their troops quickly. With better intelligence and better commanders the Germans decisively defeated the Russians. The Germans captured 92,000 Russian troops and killed another 78,000. The Russian lost 350 large guns. The Russians were forced to withdraw from most of Prussia. However without enough troops the Germans were not able to followup the victory.