On the Eastern Front, Russian armies invade East Prussia. But they blunder into disaster at the Battle of Tannenberg, where General von Hindenburg and his Chief of Staff Erich Ludendorff mastermind a brilliant German victory, taking 90,000 prisoners and destroying an entire Russian army. The Russians contribute to their own defeat by transmitting uncoded wireless messages. A second massive German victory at Masurian Lakes forces the Russians into retreat. In just six weeks, the Russian army suffers nearly a third of a million casualties.
Austro-Hungary's invasion of Serbia suffers a humiliating reverse at the Battle of Cer. Austro-Hungary's offensive against Russia also ends in disaster and retreat, with the loss of more than 300,000 men. The fortress-town of Przemyśl is cut-off and besieged by the Russians. The Germans are forced to come to the rescue, launching a diversionary attack towards Warsaw. It leads to weeks of brutal, winter fighting around the Polish city of Łódź, but there is no clear winner.