In January 1807 new Russian army commander Levin August von Bennigsen attempted to surprise the French left wing by shifting the bulk of his army north from Nowogród to East Prussia. Incorporating a Prussian corps on his right, he first bumped into elements of the VI Corps of Marshal Michel Ney, who had disobeyed his emperor's orders and advanced far north of his assigned winter cantonments. Having cleared Ney's troops out of the way, the Russians rolled down on the isolated French I Corps under Marshal Jean-Baptiste Bernadotte. Tough fighting at the Battle of Mohrungen allowed Bernadotte's corps to escape serious damage and pull back to the southwest. With his customary inventiveness, Napoléon saw an opportunity to turn the situation to his own advantage. He instructed Bernadotte to withdraw before Bennigsen's forces, and ordered the balance of the Grande Armée to strike northward. This maneuver might envelop the Russian army's left flank and cut off its retreat to the east. By a stroke of luck, a band of Cossacks captured a messenger carrying Napoleon's plans to Bernadotte and quickly forwarded the information to Gen. Pyotr Bagration. Bernadotte was left unawares and a forewarned Bennigsen immediately ordered a retreat east to Jonkowo to avoid the trap.