The battle had turned decisively in the Germans' favor. By 11 September the Russians had been pushed back to a line running from Insterburg to Angerburg in the north, with a huge flanking maneuver developing to the south. It was at this point that the threat of encirclement appeared possible. Rennenkampf ordered a general retreat toward the Russian border, which happened rapidly under the protection of a strong rear guard. It was this speed that enabled the retreating Russian troops to escape the trap Hindenburg had planned for them. The German commander had ordered his wings to quicken their march as much as possible, but a trivial accident—a rumor of a Russian counterattack—cost the Germans half a day's march, allowing the Russians to escape to the east. These reached Gumbinnen the next day, and Stallupönen on the 13th. The remains of the First Army retreated to the safety of their own border forts. Likewise, the Tenth Army was forced back into Russia. German casualties were about 10,000, Russians 100,000-125,000.