In the East, however, the war was not confined to trenches. The Russian initial plans for war had called for simultaneous invasions of Austrian Galicia and German East Prussia. Although Russia's initial advance into Galicia was largely successful, it was driven back from East Prussia by the victories of the German generals Hindenburg and Ludendorff at Tannenberg and the Masurian Lakes in August and September 1914. Russia's less-developed economic and military organisation soon proved unequal to the combined might of the German and Austro-Hungarian Empires. In the spring of 1915 the Russians were driven back in Galicia, and in May the Central Powers achieved a remarkable breakthrough on Poland's southern fringes, capturing Warsaw on 5 August and forcing the Russians to withdraw from all of Poland, known as the "Great Retreat".