In eastern Europe, the war plans also went into general operation. Two Russian armies drove into East Prussia—one from the south pushing up from Russian Poland, one from the east through the Masurian Lakes region—occupying German territory and threatening further advances. Energetic German commanders, notably General Paul von Hindenburg, his vigorous assistant General Eric Ludendorff, and the key military planner Colonel Max Hoffmann, managed to isolate and annihilate the southern army under General Alexander Samsonov. They then turned and repelled General Paul Rennenkampf's force, which was advancing westward. It seemed a more decisive victory than anything that was occurring on the western front: Samsonov's force of 300,000 was destroyed or captured. But it too left both sides basically intact to fight again. Russian forces had much greater success penetrating and occupying Galicia, the northern part of the Austro-Hungarian Empire. Their attack met advancing Austrian troops under Conrad von Hotzendorf; the collision between the two ended in Conrad's withdrawal. To add to Austria's embarrassment in the first episodes of the war, several Austrian offensives against minuscule Serbia—seen as Austria's archenemy and the cause of the war — ended in failure. An encouraging event for the Central Powers came in early November, however, when Ottoman Turkey entered the war on their side.