Two days ago the Germans attacked in the Masurian Lakes region of East Prussia, punching a hole in the Russian line. Now they launch a second wave of their attack, coming at the Russians from the north with a fresh army. The Russians reel, surprised that the Germans have so many men to attack with. With the Russians withdrawing in considerable disorder and the Germans hitting their right flank, there is a real danger that the battle will turn into a second Tannenberg, with the Russian army cut off, surrounded and destroyed.
image source (Mental Floss WW1 Centennial)